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class="vector-toc vector-pinnable-element"> <div class="vector-pinnable-header vector-toc-pinnable-header vector-pinnable-header-pinned" data-feature-name="toc-pinned" data-pinnable-element-id="vector-toc" > <h2 class="vector-pinnable-header-label">Contents</h2> <button class="vector-pinnable-header-toggle-button vector-pinnable-header-pin-button" data-event-name="pinnable-header.vector-toc.pin">move to sidebar</button> <button class="vector-pinnable-header-toggle-button vector-pinnable-header-unpin-button" data-event-name="pinnable-header.vector-toc.unpin">hide</button> </div> <ul class="vector-toc-contents" id="mw-panel-toc-list"> <li id="toc-mw-content-text" class="vector-toc-list-item vector-toc-level-1"> <a href="#" class="vector-toc-link"> <div class="vector-toc-text">Beginning</div> </a> </li> <li id="toc-Introduction" class="vector-toc-list-item vector-toc-level-1"> <a class="vector-toc-link" href="#Introduction"> <div class="vector-toc-text"> <span class="vector-toc-numb">1</span> <span>Introduction</span> </div> </a> <button aria-controls="toc-Introduction-sublist" class="cdx-button cdx-button--weight-quiet cdx-button--icon-only vector-toc-toggle"> <span class="vector-icon mw-ui-icon-wikimedia-expand"></span> <span>Toggle Introduction subsection</span> </button> <ul id="toc-Introduction-sublist" class="vector-toc-list"> <li id="toc-Difference_between_physical_Disks_and_logical_disks" class="vector-toc-list-item vector-toc-level-2"> <a class="vector-toc-link" href="#Difference_between_physical_Disks_and_logical_disks"> <div class="vector-toc-text"> <span class="vector-toc-numb">1.1</span> <span>Difference between physical Disks and logical disks</span> </div> </a> <ul id="toc-Difference_between_physical_Disks_and_logical_disks-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-Reading_and_writing_data" class="vector-toc-list-item vector-toc-level-2"> <a class="vector-toc-link" href="#Reading_and_writing_data"> <div class="vector-toc-text"> <span class="vector-toc-numb">1.2</span> <span>Reading and writing data</span> </div> </a> <ul id="toc-Reading_and_writing_data-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-What_is_RAID?" class="vector-toc-list-item vector-toc-level-2"> <a class="vector-toc-link" href="#What_is_RAID?"> <div class="vector-toc-text"> <span class="vector-toc-numb">1.3</span> <span>What is RAID?</span> </div> </a> <ul id="toc-What_is_RAID?-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-Why_use_RAID?" class="vector-toc-list-item vector-toc-level-2"> <a class="vector-toc-link" href="#Why_use_RAID?"> <div class="vector-toc-text"> <span class="vector-toc-numb">1.4</span> <span>Why use RAID?</span> </div> </a> <ul id="toc-Why_use_RAID?-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-History" class="vector-toc-list-item vector-toc-level-2"> <a class="vector-toc-link" href="#History"> <div class="vector-toc-text"> <span class="vector-toc-numb">1.5</span> <span>History</span> </div> </a> <ul id="toc-History-sublist" class="vector-toc-list"> </ul> </li> </ul> </li> <li id="toc-Basic_concepts_used_by_RAID_systems" class="vector-toc-list-item vector-toc-level-1"> <a class="vector-toc-link" href="#Basic_concepts_used_by_RAID_systems"> <div class="vector-toc-text"> <span class="vector-toc-numb">2</span> <span>Basic concepts used by RAID systems</span> </div> </a> <button aria-controls="toc-Basic_concepts_used_by_RAID_systems-sublist" class="cdx-button cdx-button--weight-quiet cdx-button--icon-only vector-toc-toggle"> <span class="vector-icon mw-ui-icon-wikimedia-expand"></span> <span>Toggle Basic concepts used by RAID systems subsection</span> </button> <ul id="toc-Basic_concepts_used_by_RAID_systems-sublist" class="vector-toc-list"> <li id="toc-Caching" class="vector-toc-list-item vector-toc-level-2"> <a class="vector-toc-link" href="#Caching"> <div class="vector-toc-text"> <span class="vector-toc-numb">2.1</span> <span>Caching</span> </div> </a> <ul id="toc-Caching-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-Mirroring:_More_than_one_copy_of_the_data" class="vector-toc-list-item vector-toc-level-2"> <a class="vector-toc-link" href="#Mirroring:_More_than_one_copy_of_the_data"> <div class="vector-toc-text"> <span class="vector-toc-numb">2.2</span> <span>Mirroring: More than one copy of the data</span> </div> </a> <ul id="toc-Mirroring:_More_than_one_copy_of_the_data-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-Striping:_Part_of_the_data_is_on_another_disk" class="vector-toc-list-item vector-toc-level-2"> <a class="vector-toc-link" href="#Striping:_Part_of_the_data_is_on_another_disk"> <div class="vector-toc-text"> <span class="vector-toc-numb">2.3</span> <span>Striping: Part of the data is on another disk</span> </div> </a> <ul id="toc-Striping:_Part_of_the_data_is_on_another_disk-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-Error_correction_and_faults" class="vector-toc-list-item vector-toc-level-2"> <a class="vector-toc-link" href="#Error_correction_and_faults"> <div class="vector-toc-text"> <span class="vector-toc-numb">2.4</span> <span>Error correction and faults</span> </div> </a> <ul id="toc-Error_correction_and_faults-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-Hot_spares:_using_more_disks_than_needed" class="vector-toc-list-item vector-toc-level-2"> <a class="vector-toc-link" href="#Hot_spares:_using_more_disks_than_needed"> <div class="vector-toc-text"> <span class="vector-toc-numb">2.5</span> <span>Hot spares: using more disks than needed</span> </div> </a> <ul id="toc-Hot_spares:_using_more_disks_than_needed-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-Stripe_size_and_chunk_size:_spreading_the_data_over_several_disks" class="vector-toc-list-item vector-toc-level-2"> <a class="vector-toc-link" href="#Stripe_size_and_chunk_size:_spreading_the_data_over_several_disks"> <div class="vector-toc-text"> <span class="vector-toc-numb">2.6</span> <span>Stripe size and chunk size: spreading the data over several disks</span> </div> </a> <ul id="toc-Stripe_size_and_chunk_size:_spreading_the_data_over_several_disks-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-Putting_disk_together:_JBOD,_concatenation_or_spanning" class="vector-toc-list-item vector-toc-level-2"> <a class="vector-toc-link" href="#Putting_disk_together:_JBOD,_concatenation_or_spanning"> <div class="vector-toc-text"> <span class="vector-toc-numb">2.7</span> <span>Putting disk together: JBOD, concatenation or spanning</span> </div> </a> <ul id="toc-Putting_disk_together:_JBOD,_concatenation_or_spanning-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-Drive_Clone" class="vector-toc-list-item vector-toc-level-2"> <a class="vector-toc-link" href="#Drive_Clone"> <div class="vector-toc-text"> <span class="vector-toc-numb">2.8</span> <span>Drive Clone</span> </div> </a> <ul id="toc-Drive_Clone-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-Different_setups" class="vector-toc-list-item vector-toc-level-2"> <a class="vector-toc-link" href="#Different_setups"> <div class="vector-toc-text"> <span class="vector-toc-numb">2.9</span> <span>Different setups</span> </div> </a> <ul id="toc-Different_setups-sublist" class="vector-toc-list"> </ul> </li> </ul> </li> <li id="toc-Basics:_simple_RAID_levels" class="vector-toc-list-item vector-toc-level-1"> <a class="vector-toc-link" href="#Basics:_simple_RAID_levels"> <div class="vector-toc-text"> <span class="vector-toc-numb">3</span> <span>Basics: simple RAID levels</span> </div> </a> <button aria-controls="toc-Basics:_simple_RAID_levels-sublist" class="cdx-button cdx-button--weight-quiet cdx-button--icon-only vector-toc-toggle"> <span class="vector-icon mw-ui-icon-wikimedia-expand"></span> <span>Toggle Basics: simple RAID levels subsection</span> </button> <ul id="toc-Basics:_simple_RAID_levels-sublist" class="vector-toc-list"> <li id="toc-RAID_levels_in_common_use" class="vector-toc-list-item vector-toc-level-2"> <a class="vector-toc-link" href="#RAID_levels_in_common_use"> <div class="vector-toc-text"> <span class="vector-toc-numb">3.1</span> <span>RAID levels in common use</span> </div> </a> <ul id="toc-RAID_levels_in_common_use-sublist" class="vector-toc-list"> <li id="toc-RAID_0_"striping"" class="vector-toc-list-item vector-toc-level-3"> <a class="vector-toc-link" href="#RAID_0_"striping""> <div class="vector-toc-text"> <span class="vector-toc-numb">3.1.1</span> <span>RAID 0 "striping"</span> </div> </a> <ul id="toc-RAID_0_"striping"-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-RAID_1_"mirroring"" class="vector-toc-list-item vector-toc-level-3"> <a class="vector-toc-link" href="#RAID_1_"mirroring""> <div class="vector-toc-text"> <span class="vector-toc-numb">3.1.2</span> <span>RAID 1 "mirroring"</span> </div> </a> <ul id="toc-RAID_1_"mirroring"-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-RAID_5_"striping_with_distributed_parity"" class="vector-toc-list-item vector-toc-level-3"> <a class="vector-toc-link" href="#RAID_5_"striping_with_distributed_parity""> <div class="vector-toc-text"> <span class="vector-toc-numb">3.1.3</span> <span>RAID 5 "striping with distributed parity"</span> </div> </a> <ul id="toc-RAID_5_"striping_with_distributed_parity"-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-Pictures" class="vector-toc-list-item vector-toc-level-3"> <a class="vector-toc-link" href="#Pictures"> <div class="vector-toc-text"> <span class="vector-toc-numb">3.1.4</span> <span>Pictures</span> </div> </a> <ul id="toc-Pictures-sublist" class="vector-toc-list"> </ul> </li> </ul> </li> <li id="toc-RAID_levels_used_less" class="vector-toc-list-item vector-toc-level-2"> <a class="vector-toc-link" href="#RAID_levels_used_less"> <div class="vector-toc-text"> <span class="vector-toc-numb">3.2</span> <span>RAID levels used less</span> </div> </a> <ul id="toc-RAID_levels_used_less-sublist" class="vector-toc-list"> <li id="toc-RAID_2" class="vector-toc-list-item vector-toc-level-3"> <a class="vector-toc-link" href="#RAID_2"> <div class="vector-toc-text"> <span class="vector-toc-numb">3.2.1</span> <span>RAID 2</span> </div> </a> <ul id="toc-RAID_2-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-RAID_3_"striping_with_dedicated_parity"" class="vector-toc-list-item vector-toc-level-3"> <a class="vector-toc-link" href="#RAID_3_"striping_with_dedicated_parity""> <div class="vector-toc-text"> <span class="vector-toc-numb">3.2.2</span> <span>RAID 3 "striping with dedicated parity"</span> </div> </a> <ul id="toc-RAID_3_"striping_with_dedicated_parity"-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-RAID_4_"striping_with_dedicated_parity"" class="vector-toc-list-item vector-toc-level-3"> <a class="vector-toc-link" href="#RAID_4_"striping_with_dedicated_parity""> <div class="vector-toc-text"> <span class="vector-toc-numb">3.2.3</span> <span>RAID 4 "striping with dedicated parity"</span> </div> </a> <ul id="toc-RAID_4_"striping_with_dedicated_parity"-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-RAID_6" class="vector-toc-list-item vector-toc-level-3"> <a class="vector-toc-link" href="#RAID_6"> <div class="vector-toc-text"> <span class="vector-toc-numb">3.2.4</span> <span>RAID 6</span> </div> </a> <ul id="toc-RAID_6-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-Pictures_2" class="vector-toc-list-item vector-toc-level-3"> <a class="vector-toc-link" href="#Pictures_2"> <div class="vector-toc-text"> <span class="vector-toc-numb">3.2.5</span> <span>Pictures</span> </div> </a> <ul id="toc-Pictures_2-sublist" class="vector-toc-list"> </ul> </li> </ul> </li> <li id="toc-Non-standard_RAID_levels" class="vector-toc-list-item vector-toc-level-2"> <a class="vector-toc-link" href="#Non-standard_RAID_levels"> <div class="vector-toc-text"> <span class="vector-toc-numb">3.3</span> <span>Non-standard RAID levels</span> </div> </a> <ul id="toc-Non-standard_RAID_levels-sublist" class="vector-toc-list"> <li id="toc-Double_parity_/_Diagonal_parity" class="vector-toc-list-item vector-toc-level-3"> <a class="vector-toc-link" href="#Double_parity_/_Diagonal_parity"> <div class="vector-toc-text"> <span class="vector-toc-numb">3.3.1</span> <span>Double parity / Diagonal parity</span> </div> </a> <ul id="toc-Double_parity_/_Diagonal_parity-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-RAID-DP" class="vector-toc-list-item vector-toc-level-3"> <a class="vector-toc-link" href="#RAID-DP"> <div class="vector-toc-text"> <span class="vector-toc-numb">3.3.2</span> <span>RAID-DP</span> </div> </a> <ul id="toc-RAID-DP-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-RAID_1.5" class="vector-toc-list-item vector-toc-level-3"> <a class="vector-toc-link" href="#RAID_1.5"> <div class="vector-toc-text"> <span class="vector-toc-numb">3.3.3</span> <span>RAID 1.5</span> </div> </a> <ul id="toc-RAID_1.5-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-RAID_5E,_RAID_5EE_and_RAID_6E" class="vector-toc-list-item vector-toc-level-3"> <a class="vector-toc-link" href="#RAID_5E,_RAID_5EE_and_RAID_6E"> <div class="vector-toc-text"> <span class="vector-toc-numb">3.3.4</span> <span>RAID 5E, RAID 5EE and RAID 6E</span> </div> </a> <ul id="toc-RAID_5E,_RAID_5EE_and_RAID_6E-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-RAID_7" class="vector-toc-list-item vector-toc-level-3"> <a class="vector-toc-link" href="#RAID_7"> <div class="vector-toc-text"> <span class="vector-toc-numb">3.3.5</span> <span>RAID 7</span> </div> </a> <ul id="toc-RAID_7-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-Intel_Matrix_RAID" class="vector-toc-list-item vector-toc-level-3"> <a class="vector-toc-link" href="#Intel_Matrix_RAID"> <div class="vector-toc-text"> <span class="vector-toc-numb">3.3.6</span> <span>Intel Matrix RAID</span> </div> </a> <ul id="toc-Intel_Matrix_RAID-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-Linux_MD_RAID_driver" class="vector-toc-list-item vector-toc-level-3"> <a class="vector-toc-link" href="#Linux_MD_RAID_driver"> <div class="vector-toc-text"> <span class="vector-toc-numb">3.3.7</span> <span>Linux MD RAID driver</span> </div> </a> <ul id="toc-Linux_MD_RAID_driver-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-RAID_Z" class="vector-toc-list-item vector-toc-level-3"> <a class="vector-toc-link" href="#RAID_Z"> <div class="vector-toc-text"> <span class="vector-toc-numb">3.3.8</span> <span>RAID Z</span> </div> </a> <ul id="toc-RAID_Z-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-Pictures_3" class="vector-toc-list-item vector-toc-level-3"> <a class="vector-toc-link" href="#Pictures_3"> <div class="vector-toc-text"> <span class="vector-toc-numb">3.3.9</span> <span>Pictures</span> </div> </a> <ul id="toc-Pictures_3-sublist" class="vector-toc-list"> </ul> </li> </ul> </li> </ul> </li> <li id="toc-Joining_RAID_levels" class="vector-toc-list-item vector-toc-level-1"> <a class="vector-toc-link" href="#Joining_RAID_levels"> <div class="vector-toc-text"> <span class="vector-toc-numb">4</span> <span>Joining RAID levels</span> </div> </a> <ul id="toc-Joining_RAID_levels-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-Making_a_RAID" class="vector-toc-list-item vector-toc-level-1"> <a class="vector-toc-link" href="#Making_a_RAID"> <div class="vector-toc-text"> <span class="vector-toc-numb">5</span> <span>Making a RAID</span> </div> </a> <button aria-controls="toc-Making_a_RAID-sublist" class="cdx-button cdx-button--weight-quiet cdx-button--icon-only vector-toc-toggle"> <span class="vector-icon mw-ui-icon-wikimedia-expand"></span> <span>Toggle Making a RAID subsection</span> </button> <ul id="toc-Making_a_RAID-sublist" class="vector-toc-list"> <li id="toc-Software_RAID" class="vector-toc-list-item vector-toc-level-2"> <a class="vector-toc-link" href="#Software_RAID"> <div class="vector-toc-text"> <span class="vector-toc-numb">5.1</span> <span>Software RAID</span> </div> </a> <ul id="toc-Software_RAID-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-Hardware_RAID" class="vector-toc-list-item vector-toc-level-2"> <a class="vector-toc-link" href="#Hardware_RAID"> <div class="vector-toc-text"> <span class="vector-toc-numb">5.2</span> <span>Hardware RAID</span> </div> </a> <ul id="toc-Hardware_RAID-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-Hardware-assisted_RAID" class="vector-toc-list-item vector-toc-level-2"> <a class="vector-toc-link" href="#Hardware-assisted_RAID"> <div class="vector-toc-text"> <span class="vector-toc-numb">5.3</span> <span>Hardware-assisted RAID</span> </div> </a> <ul id="toc-Hardware-assisted_RAID-sublist" class="vector-toc-list"> </ul> </li> </ul> </li> <li id="toc-Different_terms_related_to_hardware_failures" class="vector-toc-list-item vector-toc-level-1"> <a class="vector-toc-link" href="#Different_terms_related_to_hardware_failures"> <div class="vector-toc-text"> <span class="vector-toc-numb">6</span> <span>Different terms related to hardware failures</span> </div> </a> <button aria-controls="toc-Different_terms_related_to_hardware_failures-sublist" class="cdx-button cdx-button--weight-quiet cdx-button--icon-only vector-toc-toggle"> <span class="vector-icon mw-ui-icon-wikimedia-expand"></span> <span>Toggle Different terms related to hardware failures subsection</span> </button> <ul id="toc-Different_terms_related_to_hardware_failures-sublist" class="vector-toc-list"> <li id="toc-Failure_rate" class="vector-toc-list-item vector-toc-level-2"> <a class="vector-toc-link" href="#Failure_rate"> <div class="vector-toc-text"> <span class="vector-toc-numb">6.1</span> <span>Failure rate</span> </div> </a> <ul id="toc-Failure_rate-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-Mean_time_to_data_loss" class="vector-toc-list-item vector-toc-level-2"> <a class="vector-toc-link" href="#Mean_time_to_data_loss"> <div class="vector-toc-text"> <span class="vector-toc-numb">6.2</span> <span>Mean time to data loss</span> </div> </a> <ul id="toc-Mean_time_to_data_loss-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-Mean_time_to_recovery" class="vector-toc-list-item vector-toc-level-2"> <a class="vector-toc-link" href="#Mean_time_to_recovery"> <div class="vector-toc-text"> <span class="vector-toc-numb">6.3</span> <span>Mean time to recovery</span> </div> </a> <ul id="toc-Mean_time_to_recovery-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-Unrecoverable_bit_error_rate" class="vector-toc-list-item vector-toc-level-2"> <a class="vector-toc-link" href="#Unrecoverable_bit_error_rate"> <div class="vector-toc-text"> <span class="vector-toc-numb">6.4</span> <span>Unrecoverable bit error rate</span> </div> </a> <ul id="toc-Unrecoverable_bit_error_rate-sublist" class="vector-toc-list"> </ul> </li> </ul> </li> <li id="toc-Problems_with_RAID" class="vector-toc-list-item vector-toc-level-1"> <a class="vector-toc-link" href="#Problems_with_RAID"> <div class="vector-toc-text"> <span class="vector-toc-numb">7</span> <span>Problems with RAID</span> </div> </a> <button aria-controls="toc-Problems_with_RAID-sublist" class="cdx-button cdx-button--weight-quiet cdx-button--icon-only vector-toc-toggle"> <span class="vector-icon mw-ui-icon-wikimedia-expand"></span> <span>Toggle Problems with RAID subsection</span> </button> <ul id="toc-Problems_with_RAID-sublist" class="vector-toc-list"> <li id="toc-Adding_disks_at_a_later_time" class="vector-toc-list-item vector-toc-level-2"> <a class="vector-toc-link" href="#Adding_disks_at_a_later_time"> <div class="vector-toc-text"> <span class="vector-toc-numb">7.1</span> <span>Adding disks at a later time</span> </div> </a> <ul id="toc-Adding_disks_at_a_later_time-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-Linked_failures" class="vector-toc-list-item vector-toc-level-2"> <a class="vector-toc-link" href="#Linked_failures"> <div class="vector-toc-text"> <span class="vector-toc-numb">7.2</span> <span>Linked failures</span> </div> </a> <ul id="toc-Linked_failures-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-Atomicity" class="vector-toc-list-item vector-toc-level-2"> <a class="vector-toc-link" href="#Atomicity"> <div class="vector-toc-text"> <span class="vector-toc-numb">7.3</span> <span>Atomicity</span> </div> </a> <ul id="toc-Atomicity-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-Unrecoverable_data" class="vector-toc-list-item vector-toc-level-2"> <a class="vector-toc-link" href="#Unrecoverable_data"> <div class="vector-toc-text"> <span class="vector-toc-numb">7.4</span> <span>Unrecoverable data</span> </div> </a> <ul id="toc-Unrecoverable_data-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-Write_cache_reliability" class="vector-toc-list-item vector-toc-level-2"> <a class="vector-toc-link" href="#Write_cache_reliability"> <div class="vector-toc-text"> <span class="vector-toc-numb">7.5</span> <span>Write cache reliability</span> </div> </a> <ul id="toc-Write_cache_reliability-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-Equipment_compatibility" class="vector-toc-list-item vector-toc-level-2"> <a class="vector-toc-link" href="#Equipment_compatibility"> <div class="vector-toc-text"> <span class="vector-toc-numb">7.6</span> <span>Equipment compatibility</span> </div> </a> <ul id="toc-Equipment_compatibility-sublist" class="vector-toc-list"> </ul> </li> </ul> </li> <li id="toc-What_RAID_can_and_cannot_do" class="vector-toc-list-item vector-toc-level-1"> <a class="vector-toc-link" href="#What_RAID_can_and_cannot_do"> <div class="vector-toc-text"> <span class="vector-toc-numb">8</span> <span>What RAID can and cannot do</span> </div> </a> <button aria-controls="toc-What_RAID_can_and_cannot_do-sublist" class="cdx-button cdx-button--weight-quiet cdx-button--icon-only vector-toc-toggle"> <span class="vector-icon mw-ui-icon-wikimedia-expand"></span> <span>Toggle What RAID can and cannot do subsection</span> </button> <ul id="toc-What_RAID_can_and_cannot_do-sublist" class="vector-toc-list"> <li id="toc-What_RAID_can_do" class="vector-toc-list-item vector-toc-level-2"> <a class="vector-toc-link" href="#What_RAID_can_do"> <div class="vector-toc-text"> <span class="vector-toc-numb">8.1</span> <span>What RAID can do</span> </div> </a> <ul id="toc-What_RAID_can_do-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-What_RAID_cannot_do" class="vector-toc-list-item vector-toc-level-2"> <a class="vector-toc-link" href="#What_RAID_cannot_do"> <div class="vector-toc-text"> <span class="vector-toc-numb">8.2</span> <span>What RAID cannot do</span> </div> </a> <ul id="toc-What_RAID_cannot_do-sublist" class="vector-toc-list"> </ul> </li> </ul> </li> <li id="toc-Example" class="vector-toc-list-item vector-toc-level-1"> <a class="vector-toc-link" href="#Example"> <div class="vector-toc-text"> <span class="vector-toc-numb">9</span> <span>Example</span> </div> </a> <ul id="toc-Example-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-References" class="vector-toc-list-item vector-toc-level-1"> <a class="vector-toc-link" href="#References"> <div class="vector-toc-text"> <span class="vector-toc-numb">10</span> <span>References</span> </div> </a> <ul id="toc-References-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-Other_websites" class="vector-toc-list-item vector-toc-level-1"> <a class="vector-toc-link" href="#Other_websites"> <div class="vector-toc-text"> <span class="vector-toc-numb">11</span> <span>Other websites</span> </div> </a> <ul id="toc-Other_websites-sublist" class="vector-toc-list"> </ul> </li> </ul> </div> </div> </nav> </div> </div> <div class="mw-content-container"> <main id="content" class="mw-body"> <header class="mw-body-header vector-page-titlebar"> <nav aria-label="Contents" class="vector-toc-landmark"> <div id="vector-page-titlebar-toc" class="vector-dropdown vector-page-titlebar-toc vector-button-flush-left" title="Table of Contents" > <input type="checkbox" id="vector-page-titlebar-toc-checkbox" role="button" aria-haspopup="true" data-event-name="ui.dropdown-vector-page-titlebar-toc" class="vector-dropdown-checkbox " aria-label="Toggle the table of contents" > <label id="vector-page-titlebar-toc-label" for="vector-page-titlebar-toc-checkbox" class="vector-dropdown-label cdx-button cdx-button--fake-button cdx-button--fake-button--enabled cdx-button--weight-quiet cdx-button--icon-only " aria-hidden="true" ><span class="vector-icon mw-ui-icon-listBullet mw-ui-icon-wikimedia-listBullet"></span> <span class="vector-dropdown-label-text">Toggle the table of contents</span> </label> <div class="vector-dropdown-content"> <div id="vector-page-titlebar-toc-unpinned-container" class="vector-unpinned-container"> </div> </div> </div> </nav> <h1 id="firstHeading" class="firstHeading mw-first-heading"><span class="mw-page-title-main">RAID</span></h1> <div id="p-lang-btn" class="vector-dropdown mw-portlet mw-portlet-lang" > <input type="checkbox" id="p-lang-btn-checkbox" role="button" aria-haspopup="true" data-event-name="ui.dropdown-p-lang-btn" class="vector-dropdown-checkbox mw-interlanguage-selector" aria-label="Go to an article in another language. Available in 50 languages" > <label id="p-lang-btn-label" for="p-lang-btn-checkbox" class="vector-dropdown-label cdx-button cdx-button--fake-button cdx-button--fake-button--enabled cdx-button--weight-quiet cdx-button--action-progressive mw-portlet-lang-heading-50" aria-hidden="true" ><span class="vector-icon mw-ui-icon-language-progressive mw-ui-icon-wikimedia-language-progressive"></span> <span class="vector-dropdown-label-text">50 languages</span> </label> <div class="vector-dropdown-content"> <div class="vector-menu-content"> <ul class="vector-menu-content-list"> <li class="interlanguage-link interwiki-af mw-list-item"><a href="https://af.wikipedia.org/wiki/RAID" title="RAID – Afrikaans" lang="af" hreflang="af" data-title="RAID" data-language-autonym="Afrikaans" data-language-local-name="Afrikaans" class="interlanguage-link-target"><span>Afrikaans</span></a></li><li class="interlanguage-link interwiki-ar mw-list-item"><a href="https://ar.wikipedia.org/wiki/%D9%85%D8%B5%D9%81%D9%88%D9%81%D8%A9_%D9%81%D8%A7%D8%A6%D8%B6%D8%A9_%D9%85%D8%B3%D8%AA%D9%82%D9%84%D8%A9_%D8%A7%D9%84%D8%A3%D9%82%D8%B1%D8%A7%D8%B5" title="مصفوفة فائضة مستقلة الأقراص – Arabic" lang="ar" hreflang="ar" data-title="مصفوفة فائضة مستقلة الأقراص" data-language-autonym="العربية" data-language-local-name="Arabic" class="interlanguage-link-target"><span>العربية</span></a></li><li class="interlanguage-link interwiki-bg mw-list-item"><a href="https://bg.wikipedia.org/wiki/RAID" title="RAID – Bulgarian" lang="bg" hreflang="bg" data-title="RAID" data-language-autonym="Български" data-language-local-name="Bulgarian" class="interlanguage-link-target"><span>Български</span></a></li><li class="interlanguage-link interwiki-ca mw-list-item"><a href="https://ca.wikipedia.org/wiki/RAID" title="RAID – Catalan" lang="ca" hreflang="ca" data-title="RAID" data-language-autonym="Català" data-language-local-name="Catalan" class="interlanguage-link-target"><span>Català</span></a></li><li class="interlanguage-link interwiki-cs mw-list-item"><a href="https://cs.wikipedia.org/wiki/RAID" title="RAID – Czech" lang="cs" hreflang="cs" data-title="RAID" data-language-autonym="Čeština" data-language-local-name="Czech" class="interlanguage-link-target"><span>Čeština</span></a></li><li class="interlanguage-link interwiki-da mw-list-item"><a href="https://da.wikipedia.org/wiki/RAID" title="RAID – Danish" lang="da" hreflang="da" data-title="RAID" data-language-autonym="Dansk" data-language-local-name="Danish" class="interlanguage-link-target"><span>Dansk</span></a></li><li class="interlanguage-link interwiki-de badge-Q17437798 badge-goodarticle mw-list-item" title="good article badge"><a href="https://de.wikipedia.org/wiki/RAID" title="RAID – German" lang="de" hreflang="de" data-title="RAID" data-language-autonym="Deutsch" data-language-local-name="German" class="interlanguage-link-target"><span>Deutsch</span></a></li><li class="interlanguage-link interwiki-et mw-list-item"><a href="https://et.wikipedia.org/wiki/S%C3%B5ltumatute_ketaste_liiasmassiiv" title="Sõltumatute ketaste liiasmassiiv – Estonian" lang="et" hreflang="et" data-title="Sõltumatute ketaste liiasmassiiv" data-language-autonym="Eesti" data-language-local-name="Estonian" class="interlanguage-link-target"><span>Eesti</span></a></li><li class="interlanguage-link interwiki-el mw-list-item"><a href="https://el.wikipedia.org/wiki/RAID" title="RAID – Greek" lang="el" hreflang="el" data-title="RAID" data-language-autonym="Ελληνικά" data-language-local-name="Greek" class="interlanguage-link-target"><span>Ελληνικά</span></a></li><li class="interlanguage-link interwiki-en mw-list-item"><a href="https://en.wikipedia.org/wiki/RAID" title="RAID – English" lang="en" hreflang="en" data-title="RAID" data-language-autonym="English" data-language-local-name="English" class="interlanguage-link-target"><span>English</span></a></li><li class="interlanguage-link interwiki-es mw-list-item"><a href="https://es.wikipedia.org/wiki/RAID" title="RAID – Spanish" lang="es" hreflang="es" data-title="RAID" data-language-autonym="Español" data-language-local-name="Spanish" class="interlanguage-link-target"><span>Español</span></a></li><li class="interlanguage-link interwiki-eo mw-list-item"><a href="https://eo.wikipedia.org/wiki/RAID" title="RAID – Esperanto" lang="eo" hreflang="eo" data-title="RAID" data-language-autonym="Esperanto" data-language-local-name="Esperanto" class="interlanguage-link-target"><span>Esperanto</span></a></li><li class="interlanguage-link interwiki-eu mw-list-item"><a href="https://eu.wikipedia.org/wiki/RAID_(informatika)" title="RAID (informatika) – Basque" lang="eu" hreflang="eu" data-title="RAID (informatika)" data-language-autonym="Euskara" data-language-local-name="Basque" class="interlanguage-link-target"><span>Euskara</span></a></li><li class="interlanguage-link interwiki-fa mw-list-item"><a href="https://fa.wikipedia.org/wiki/%D8%A2%D8%B1%D8%A7%DB%8C%D9%87_%DA%86%D9%86%D8%AF%DA%AF%D8%A7%D9%86%D9%87_%D8%AF%DB%8C%D8%B3%DA%A9%E2%80%8C%D9%87%D8%A7%DB%8C_%D9%85%D8%B3%D8%AA%D9%82%D9%84" title="آرایه چندگانه دیسک‌های مستقل – Persian" lang="fa" hreflang="fa" data-title="آرایه چندگانه دیسک‌های مستقل" data-language-autonym="فارسی" data-language-local-name="Persian" class="interlanguage-link-target"><span>فارسی</span></a></li><li class="interlanguage-link interwiki-fr mw-list-item"><a href="https://fr.wikipedia.org/wiki/RAID_(informatique)" title="RAID (informatique) – French" lang="fr" hreflang="fr" data-title="RAID (informatique)" data-language-autonym="Français" data-language-local-name="French" class="interlanguage-link-target"><span>Français</span></a></li><li class="interlanguage-link interwiki-ko mw-list-item"><a href="https://ko.wikipedia.org/wiki/RAID" title="RAID – Korean" lang="ko" hreflang="ko" data-title="RAID" data-language-autonym="한국어" data-language-local-name="Korean" class="interlanguage-link-target"><span>한국어</span></a></li><li class="interlanguage-link interwiki-hr mw-list-item"><a href="https://hr.wikipedia.org/wiki/RAID" title="RAID – Croatian" lang="hr" hreflang="hr" data-title="RAID" data-language-autonym="Hrvatski" data-language-local-name="Croatian" class="interlanguage-link-target"><span>Hrvatski</span></a></li><li class="interlanguage-link interwiki-id mw-list-item"><a href="https://id.wikipedia.org/wiki/RAID" title="RAID – Indonesian" lang="id" hreflang="id" data-title="RAID" data-language-autonym="Bahasa Indonesia" data-language-local-name="Indonesian" class="interlanguage-link-target"><span>Bahasa Indonesia</span></a></li><li class="interlanguage-link interwiki-it mw-list-item"><a href="https://it.wikipedia.org/wiki/RAID" title="RAID – Italian" lang="it" hreflang="it" data-title="RAID" data-language-autonym="Italiano" data-language-local-name="Italian" class="interlanguage-link-target"><span>Italiano</span></a></li><li class="interlanguage-link interwiki-he mw-list-item"><a href="https://he.wikipedia.org/wiki/RAID" title="RAID – Hebrew" lang="he" hreflang="he" data-title="RAID" data-language-autonym="עברית" data-language-local-name="Hebrew" class="interlanguage-link-target"><span>עברית</span></a></li><li class="interlanguage-link interwiki-kk mw-list-item"><a href="https://kk.wikipedia.org/wiki/RAID" title="RAID – Kazakh" lang="kk" hreflang="kk" data-title="RAID" data-language-autonym="Қазақша" data-language-local-name="Kazakh" class="interlanguage-link-target"><span>Қазақша</span></a></li><li class="interlanguage-link interwiki-ky mw-list-item"><a href="https://ky.wikipedia.org/wiki/RAID_%D1%81%D0%B0%D0%BA%D1%82%D0%BE%D0%BE_%D1%81%D0%B8%D1%81%D1%82%D0%B5%D0%BC%D0%B8" title="RAID сактоо системи – Kyrgyz" lang="ky" hreflang="ky" data-title="RAID сактоо системи" data-language-autonym="Кыргызча" data-language-local-name="Kyrgyz" class="interlanguage-link-target"><span>Кыргызча</span></a></li><li class="interlanguage-link interwiki-lv mw-list-item"><a href="https://lv.wikipedia.org/wiki/RAID" title="RAID – Latvian" lang="lv" hreflang="lv" data-title="RAID" data-language-autonym="Latviešu" data-language-local-name="Latvian" class="interlanguage-link-target"><span>Latviešu</span></a></li><li class="interlanguage-link interwiki-lt mw-list-item"><a href="https://lt.wikipedia.org/wiki/RAID" title="RAID – Lithuanian" lang="lt" hreflang="lt" data-title="RAID" data-language-autonym="Lietuvių" data-language-local-name="Lithuanian" class="interlanguage-link-target"><span>Lietuvių</span></a></li><li class="interlanguage-link interwiki-lmo mw-list-item"><a href="https://lmo.wikipedia.org/wiki/RAID" title="RAID – Lombard" lang="lmo" hreflang="lmo" data-title="RAID" data-language-autonym="Lombard" data-language-local-name="Lombard" class="interlanguage-link-target"><span>Lombard</span></a></li><li class="interlanguage-link interwiki-hu mw-list-item"><a href="https://hu.wikipedia.org/wiki/RAID" title="RAID – Hungarian" lang="hu" hreflang="hu" data-title="RAID" data-language-autonym="Magyar" data-language-local-name="Hungarian" class="interlanguage-link-target"><span>Magyar</span></a></li><li class="interlanguage-link interwiki-mk mw-list-item"><a href="https://mk.wikipedia.org/wiki/RAID" title="RAID – Macedonian" lang="mk" hreflang="mk" data-title="RAID" data-language-autonym="Македонски" data-language-local-name="Macedonian" class="interlanguage-link-target"><span>Македонски</span></a></li><li class="interlanguage-link interwiki-ml mw-list-item"><a href="https://ml.wikipedia.org/wiki/%E0%B4%B1%E0%B4%AF%E0%B4%BF%E0%B4%A1%E0%B5%8D" title="റയിഡ് – Malayalam" lang="ml" hreflang="ml" data-title="റയിഡ്" data-language-autonym="മലയാളം" data-language-local-name="Malayalam" class="interlanguage-link-target"><span>മലയാളം</span></a></li><li class="interlanguage-link interwiki-ms mw-list-item"><a href="https://ms.wikipedia.org/wiki/RAID" title="RAID – Malay" lang="ms" hreflang="ms" data-title="RAID" data-language-autonym="Bahasa Melayu" data-language-local-name="Malay" class="interlanguage-link-target"><span>Bahasa Melayu</span></a></li><li class="interlanguage-link interwiki-mn mw-list-item"><a href="https://mn.wikipedia.org/wiki/RAID" title="RAID – Mongolian" lang="mn" hreflang="mn" data-title="RAID" data-language-autonym="Монгол" data-language-local-name="Mongolian" class="interlanguage-link-target"><span>Монгол</span></a></li><li class="interlanguage-link interwiki-nl mw-list-item"><a href="https://nl.wikipedia.org/wiki/Redundant_array_of_independent_disks" title="Redundant array of independent disks – Dutch" lang="nl" hreflang="nl" data-title="Redundant array of independent disks" data-language-autonym="Nederlands" data-language-local-name="Dutch" class="interlanguage-link-target"><span>Nederlands</span></a></li><li class="interlanguage-link interwiki-ja mw-list-item"><a href="https://ja.wikipedia.org/wiki/RAID" title="RAID – Japanese" lang="ja" hreflang="ja" data-title="RAID" data-language-autonym="日本語" data-language-local-name="Japanese" class="interlanguage-link-target"><span>日本語</span></a></li><li class="interlanguage-link interwiki-no mw-list-item"><a href="https://no.wikipedia.org/wiki/RAID" title="RAID – Norwegian Bokmål" lang="nb" hreflang="nb" data-title="RAID" data-language-autonym="Norsk bokmål" data-language-local-name="Norwegian Bokmål" class="interlanguage-link-target"><span>Norsk bokmål</span></a></li><li class="interlanguage-link interwiki-oc mw-list-item"><a href="https://oc.wikipedia.org/wiki/RAID_(informatica)" title="RAID (informatica) – Occitan" lang="oc" hreflang="oc" data-title="RAID (informatica)" data-language-autonym="Occitan" data-language-local-name="Occitan" class="interlanguage-link-target"><span>Occitan</span></a></li><li class="interlanguage-link interwiki-pms mw-list-item"><a href="https://pms.wikipedia.org/wiki/RAID" title="RAID – Piedmontese" lang="pms" hreflang="pms" data-title="RAID" data-language-autonym="Piemontèis" data-language-local-name="Piedmontese" class="interlanguage-link-target"><span>Piemontèis</span></a></li><li class="interlanguage-link interwiki-pl mw-list-item"><a href="https://pl.wikipedia.org/wiki/RAID" title="RAID – Polish" lang="pl" hreflang="pl" data-title="RAID" data-language-autonym="Polski" data-language-local-name="Polish" class="interlanguage-link-target"><span>Polski</span></a></li><li class="interlanguage-link interwiki-pt mw-list-item"><a href="https://pt.wikipedia.org/wiki/RAID" title="RAID – Portuguese" lang="pt" hreflang="pt" data-title="RAID" data-language-autonym="Português" data-language-local-name="Portuguese" class="interlanguage-link-target"><span>Português</span></a></li><li class="interlanguage-link interwiki-ro mw-list-item"><a href="https://ro.wikipedia.org/wiki/RAID" title="RAID – Romanian" lang="ro" hreflang="ro" data-title="RAID" data-language-autonym="Română" data-language-local-name="Romanian" class="interlanguage-link-target"><span>Română</span></a></li><li class="interlanguage-link interwiki-ru mw-list-item"><a href="https://ru.wikipedia.org/wiki/RAID" title="RAID – Russian" lang="ru" hreflang="ru" data-title="RAID" data-language-autonym="Русский" data-language-local-name="Russian" class="interlanguage-link-target"><span>Русский</span></a></li><li class="interlanguage-link interwiki-sq mw-list-item"><a href="https://sq.wikipedia.org/wiki/RAID" title="RAID – Albanian" lang="sq" hreflang="sq" data-title="RAID" data-language-autonym="Shqip" data-language-local-name="Albanian" class="interlanguage-link-target"><span>Shqip</span></a></li><li class="interlanguage-link interwiki-sk mw-list-item"><a href="https://sk.wikipedia.org/wiki/RAID" title="RAID – Slovak" lang="sk" hreflang="sk" data-title="RAID" data-language-autonym="Slovenčina" data-language-local-name="Slovak" class="interlanguage-link-target"><span>Slovenčina</span></a></li><li class="interlanguage-link interwiki-sl mw-list-item"><a href="https://sl.wikipedia.org/wiki/RAID" title="RAID – Slovenian" lang="sl" hreflang="sl" data-title="RAID" data-language-autonym="Slovenščina" data-language-local-name="Slovenian" class="interlanguage-link-target"><span>Slovenščina</span></a></li><li class="interlanguage-link interwiki-sr mw-list-item"><a href="https://sr.wikipedia.org/wiki/RAID" title="RAID – Serbian" lang="sr" hreflang="sr" data-title="RAID" data-language-autonym="Српски / srpski" data-language-local-name="Serbian" class="interlanguage-link-target"><span>Српски / srpski</span></a></li><li class="interlanguage-link interwiki-fi mw-list-item"><a href="https://fi.wikipedia.org/wiki/RAID_(tietotekniikka)" title="RAID (tietotekniikka) – Finnish" lang="fi" hreflang="fi" data-title="RAID (tietotekniikka)" data-language-autonym="Suomi" data-language-local-name="Finnish" class="interlanguage-link-target"><span>Suomi</span></a></li><li class="interlanguage-link interwiki-sv mw-list-item"><a href="https://sv.wikipedia.org/wiki/Raid" title="Raid – Swedish" lang="sv" hreflang="sv" data-title="Raid" data-language-autonym="Svenska" data-language-local-name="Swedish" class="interlanguage-link-target"><span>Svenska</span></a></li><li class="interlanguage-link interwiki-th mw-list-item"><a href="https://th.wikipedia.org/wiki/%E0%B9%80%E0%B8%A3%E0%B8%94" title="เรด – Thai" lang="th" hreflang="th" data-title="เรด" data-language-autonym="ไทย" data-language-local-name="Thai" class="interlanguage-link-target"><span>ไทย</span></a></li><li class="interlanguage-link interwiki-tr mw-list-item"><a href="https://tr.wikipedia.org/wiki/RAID" title="RAID – Turkish" lang="tr" hreflang="tr" data-title="RAID" data-language-autonym="Türkçe" data-language-local-name="Turkish" class="interlanguage-link-target"><span>Türkçe</span></a></li><li class="interlanguage-link interwiki-uk mw-list-item"><a href="https://uk.wikipedia.org/wiki/RAID" title="RAID – Ukrainian" lang="uk" hreflang="uk" data-title="RAID" data-language-autonym="Українська" data-language-local-name="Ukrainian" class="interlanguage-link-target"><span>Українська</span></a></li><li class="interlanguage-link interwiki-vi mw-list-item"><a href="https://vi.wikipedia.org/wiki/RAID" title="RAID – Vietnamese" lang="vi" hreflang="vi" data-title="RAID" data-language-autonym="Tiếng Việt" data-language-local-name="Vietnamese" class="interlanguage-link-target"><span>Tiếng Việt</span></a></li><li class="interlanguage-link interwiki-zh mw-list-item"><a href="https://zh.wikipedia.org/wiki/RAID" title="RAID – Chinese" lang="zh" hreflang="zh" data-title="RAID" data-language-autonym="中文" data-language-local-name="Chinese" class="interlanguage-link-target"><span>中文</span></a></li> </ul> <div class="after-portlet 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data-mw-ve-target-container> <div class="vector-body-before-content"> <div class="mw-indicators"> </div> <div id="siteSub" class="noprint">From Simple English Wikipedia, the free encyclopedia</div> </div> <div id="contentSub"><div id="mw-content-subtitle"></div></div> <div id="mw-content-text" class="mw-body-content"><div class="mw-content-ltr mw-parser-output" lang="en" dir="ltr"><style data-mw-deduplicate="TemplateStyles:r8036054">@media all and (max-width:720px){body.skin-minerva .mw-parser-output .tocright{display:none}.mw-parser-output .tocright{width:100%!important}}@media all and (min-width:720px){.mw-parser-output .tocright{float:right;clear:right;width:auto;margin:0 0 0.5em 1em}.mw-parser-output .tocright-clear-left{clear:left}.mw-parser-output .tocright-clear-both{clear:both}.mw-parser-output .tocright-clear-none{clear:none}}</style><div class="tocright"><meta property="mw:PageProp/toc" /></div> <style data-mw-deduplicate="TemplateStyles:r9671799">.mw-parser-output .hatnote{font-style:italic}.mw-parser-output div.hatnote{padding-left:1.6em;margin-bottom:0.5em}.mw-parser-output .hatnote i{font-style:normal}.mw-parser-output .hatnote+link+.hatnote{margin-top:-0.5em}@media print{body.ns-0 .mw-parser-output .hatnote{display:none!important}}</style><div role="note" class="hatnote navigation-not-searchable">For the military term, see <a href="/wiki/Raid" title="Raid">raid</a>.</div> <p><b>RAID</b> is an <a href="/wiki/Acronym" title="Acronym">acronym</a> that stands for <b>Redundant Array of Inexpensive</b> <b>Disks</b> or <b>Redundant Array of Independent Disks</b>. RAID is a term used in <a href="/wiki/Computer" title="Computer">computing</a>. With RAID, several <a href="/wiki/Hard_disk" title="Hard disk">hard disks</a> are made into one <a href="/wiki/Logical_disk" title="Logical disk">logical disk</a>. There are different ways this can be done. Each of the methods that puts the hard disks together has some benefits and drawbacks over using the drives as single disks, independent of each other. The main reasons why RAID is used are: </p> <ul><li>To make the loss of <a href="/wiki/Information" title="Information">data</a> happen less often. This is done by having several copies of the data.</li> <li>To get more storage space by having many smaller disks.</li> <li>To get more flexibility (Disks can be changed or added while the system keeps running)</li> <li>To get the data more quickly.</li></ul> <p>It is not possible to achieve all of these goals at the same time, so choices need to be made. </p><p>There are also some bad things: </p> <ul><li>Certain choices can protect against data being lost because one (or a number) of disks failed. They do not protect against the data being deleted or overwritten, though.</li> <li>In some configurations, RAID can tolerate that one or a number of disks fail. After the failed disks have been replaced, the data needs to be reconstructed. Depending on the configuration and the size of the disks, this reconstruction can take a long time.</li> <li>Certain kinds of errors will make it impossible to read the data</li></ul> <p>Most of the work on RAID is based on a paper written in 1988.<sup id="cite_ref-Patterson88_1-0" class="reference"><a href="#cite_note-Patterson88-1"><span class="cite-bracket">[</span>1<span class="cite-bracket">]</span></a></sup> </p><p>Companies have used RAID systems to store their data since the technology was made. There are different ways in which RAID systems can be made. Since its discovery, the cost of building a RAID system has come down a lot. For this reason, even some <a href="/wiki/Computer" title="Computer">computers</a> and <a href="/wiki/Appliance" title="Appliance">appliances</a> that are used at home have some RAID functions. Such systems can be used to store <a href="/wiki/Music" title="Music">music</a> or <a href="/wiki/Movie" title="Movie">movies</a>, for example. </p> <div class="mw-heading mw-heading2"><h2 id="Introduction">Introduction</h2><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=RAID&veaction=edit&section=1" title="Change section: Introduction" class="mw-editsection-visualeditor"><span>change</span></a><span class="mw-editsection-divider"> | </span><a href="/w/index.php?title=RAID&action=edit&section=1" title="Edit section's source code: Introduction"><span>change source</span></a><span class="mw-editsection-bracket">]</span></span></div> <div class="mw-heading mw-heading3"><h3 id="Difference_between_physical_Disks_and_logical_disks">Difference between physical Disks and logical disks</h3><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=RAID&veaction=edit&section=2" title="Change section: Difference between physical Disks and logical disks" class="mw-editsection-visualeditor"><span>change</span></a><span class="mw-editsection-divider"> | </span><a href="/w/index.php?title=RAID&action=edit&section=2" title="Edit section's source code: Difference between physical Disks and logical disks"><span>change source</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>A <a href="/wiki/Hard_disk" title="Hard disk">hard disk</a> is a part of a computer. Normal hard disks use <a href="/wiki/Magnetism" title="Magnetism">magnetism</a> to store information. When hard disks are used, they are available to the <a href="/wiki/Operating_system" title="Operating system">operating system</a>. In <a href="/wiki/Microsoft_Windows" title="Microsoft Windows">Microsoft Windows</a>, each hard disk will get a drive letter (starting with C:, A: or B: are reserved for <a href="/wiki/Floppy_drive" class="mw-redirect" title="Floppy drive">floppy drives</a>). <a href="/wiki/Unix" class="mw-redirect" title="Unix">Unix</a> and <a href="/wiki/Linux" title="Linux">Linux</a>-like operating systems have a single-rooted directory tree. This means that people who use the computers sometimes do not know where the <a href="/wiki/Information" title="Information">information</a> is stored. </p><p>In computing, the hard disks (which are hardware, and can be touched) are sometimes called <i>physical drives</i> or <i>physical disks</i>. What the operating system shows the user is sometimes called <i>logical disk</i>. A physical drive can be split into different sections, called <a href="/wiki/Disk_partitioning" title="Disk partitioning">disk partitions</a>. Usually, each disk partition contains one <a href="/wiki/File_system" title="File system">file system</a>. The operating system will show each partition like a logical disk. </p><p>Therefore, to the user, both the setup with many physical disks and the setup with many logical disks will look the same. The user cannot decide if a "logical disk" is the same as a physical disk, or if it simply is a part of the disk. <a href="/wiki/Storage_Area_Network" class="mw-redirect" title="Storage Area Network">Storage Area Networks</a> (SANs) completely change this view. All that is visible of a SAN is a number of logical disks. </p> <div class="mw-heading mw-heading3"><h3 id="Reading_and_writing_data">Reading and writing data</h3><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=RAID&veaction=edit&section=3" title="Change section: Reading and writing data" class="mw-editsection-visualeditor"><span>change</span></a><span class="mw-editsection-divider"> | </span><a href="/w/index.php?title=RAID&action=edit&section=3" title="Edit section's source code: Reading and writing data"><span>change source</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>In the computer, data is organised in the form of <a href="/wiki/Bit" class="mw-disambig" title="Bit">bits</a> and <a href="/wiki/Byte" title="Byte">bytes</a>. In most systems, 8 bits make up a byte. <a href="/wiki/Computer_memory" title="Computer memory">Computer memory</a> uses <a href="/wiki/Electricity" title="Electricity">electricity</a> to store the data, hard disks use <a href="/wiki/Magnetism" title="Magnetism">magnetism</a>. Therefore, when data is <i>written</i> on a disk, the electric signal is converted into a magnetic one. When data is read from disk, the conversion is done in the other direction: An electrical signal is made from the <a href="/wiki/Polarity" title="Polarity">polarity</a> of a <a href="/wiki/Magnetic_field" title="Magnetic field">magnetic field</a>. </p> <div class="mw-heading mw-heading3"><h3 id="What_is_RAID?"><span id="What_is_RAID.3F"></span>What is RAID?</h3><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=RAID&veaction=edit&section=4" title="Change section: What is RAID?" class="mw-editsection-visualeditor"><span>change</span></a><span class="mw-editsection-divider"> | </span><a href="/w/index.php?title=RAID&action=edit&section=4" title="Edit section's source code: What is RAID?"><span>change source</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>A RAID array joins two or more <a href="/wiki/Hard_disk" title="Hard disk">hard disks</a> so that they make a logical disk. There are different reasons why this is done. The most common ones are: </p> <ul><li>Stopping <a href="/wiki/Information" title="Information">data</a> loss, when one or more disks of the array <a class="mw-selflink-fragment" href="#RAID_level">fail</a>.</li> <li>Getting faster data transfers.</li> <li>Getting the ability to change disks while the system keeps running.</li> <li>Joining several disks to get more storage capacity; sometimes lots of cheap disks are used, rather than a more expensive one.</li></ul> <p>RAID is done by using special <a href="/wiki/Computer_hardware" title="Computer hardware">hardware</a> or <a href="/wiki/Software" title="Software">software</a> on the <a href="/wiki/Computer" title="Computer">computer</a>. The joined hard disks will then look like one hard disk to the user. Most RAID levels increase the <a href="/wiki/Redundancy" title="Redundancy">redundancy</a>. This means that they store the data more often, or they store information on how to reconstruct the data. This allows for a number of disks to fail without the data being lost. When the failed disk is replaced, the data it should contain will be copied or rebuilt from the other disks of the system. This can take a long time. The time it takes depends on different factors, like the size of the array. </p> <div class="mw-heading mw-heading3"><h3 id="Why_use_RAID?"><span id="Why_use_RAID.3F"></span>Why use RAID?</h3><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=RAID&veaction=edit&section=5" title="Change section: Why use RAID?" class="mw-editsection-visualeditor"><span>change</span></a><span class="mw-editsection-divider"> | </span><a href="/w/index.php?title=RAID&action=edit&section=5" title="Edit section's source code: Why use RAID?"><span>change source</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>One of the reasons why many companies are using RAID is that the data in the array can simply be used. Those using the data need not be aware they are using RAID at all. When a failure occurred and the array is recovering, access to the data will be slower. Accessing the data during this time will also slow down the recovery process, but this is still much faster than not being able to work with the data at all. Depending on the RAID level however, disks may not fail while the new disk is being prepared for use. A disk failing at that time will result in losing all the data in the array. </p><p>The different ways to join disks are called <i>RAID levels</i>. A bigger number for the level is not necessarily better. Different RAID levels have different purposes. Some RAID levels need special disks and special controllers. </p> <figure class="mw-default-size" typeof="mw:File/Thumb"><a href="/wiki/File:Raid_array.jpg" class="mw-file-description"><img src="//upload.wikimedia.org/wikipedia/commons/thumb/f/fb/Raid_array.jpg/220px-Raid_array.jpg" decoding="async" width="220" height="219" class="mw-file-element" srcset="//upload.wikimedia.org/wikipedia/commons/thumb/f/fb/Raid_array.jpg/330px-Raid_array.jpg 1.5x, //upload.wikimedia.org/wikipedia/commons/thumb/f/fb/Raid_array.jpg/440px-Raid_array.jpg 2x" data-file-width="720" data-file-height="716" /></a><figcaption>Close up of 4 disks that make a RAID array, built into a desktop system. The blue <a href="/wiki/Latch" title="Latch">latches</a> allow for the exchange of the disks while the system keeps running.</figcaption></figure> <figure class="mw-default-size" typeof="mw:File/Thumb"><a href="/wiki/File:RAID_single_disk.jpg" class="mw-file-description"><img src="//upload.wikimedia.org/wikipedia/commons/thumb/a/ab/RAID_single_disk.jpg/220px-RAID_single_disk.jpg" decoding="async" width="220" height="147" class="mw-file-element" srcset="//upload.wikimedia.org/wikipedia/commons/thumb/a/ab/RAID_single_disk.jpg/330px-RAID_single_disk.jpg 1.5x, //upload.wikimedia.org/wikipedia/commons/thumb/a/ab/RAID_single_disk.jpg/440px-RAID_single_disk.jpg 2x" data-file-width="968" data-file-height="648" /></a><figcaption>A hard disk has been removed from the array.</figcaption></figure> <div class="mw-heading mw-heading3"><h3 id="History">History</h3><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=RAID&veaction=edit&section=6" title="Change section: History" class="mw-editsection-visualeditor"><span>change</span></a><span class="mw-editsection-divider"> | </span><a href="/w/index.php?title=RAID&action=edit&section=6" title="Edit section's source code: History"><span>change source</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>In 1978, a man called Norman Ken Ouchi, who worked at <a href="/wiki/IBM" title="IBM">IBM</a>, made a suggestion describing the plans for what would later become RAID 5. The plans also described something similar to RAID 1, as well as the protection of a part of RAID 4. </p><p>Workers at the <a href="/wiki/University_of_Berkeley" class="mw-redirect" title="University of Berkeley">University of Berkeley</a> helped to plan out research in 1987. They were trying to make it possible for RAID technology to recognize two hard drives instead of one. They found that when RAID technology had two hard drives, it had much better storage than with only one hard drive. However, it crashed much more often. </p><p>In 1988, the different types of RAID (1 to 5), were written about by David Patterson, Garth Gibson and Randy Katz in their article, called "A Case for Redundant Arrays of Inexpensive Disks (RAID)".<sup id="cite_ref-Patterson88_1-1" class="reference"><a href="#cite_note-Patterson88-1"><span class="cite-bracket">[</span>1<span class="cite-bracket">]</span></a></sup> This article was the first to call the new technology RAID and the name became official. </p> <div class="mw-heading mw-heading2"><h2 id="Basic_concepts_used_by_RAID_systems">Basic concepts used by RAID systems</h2><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=RAID&veaction=edit&section=7" title="Change section: Basic concepts used by RAID systems" class="mw-editsection-visualeditor"><span>change</span></a><span class="mw-editsection-divider"> | </span><a href="/w/index.php?title=RAID&action=edit&section=7" title="Edit section's source code: Basic concepts used by RAID systems"><span>change source</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>RAID uses a few basic ideas, which were described in the article "RAID: High-Performance, Reliable Secondary Storage" by Peter Chen and others, published in 1994.<sup id="cite_ref-chen_2-0" class="reference"><a href="#cite_note-chen-2"><span class="cite-bracket">[</span>2<span class="cite-bracket">]</span></a></sup> </p> <div class="mw-heading mw-heading3"><h3 id="Caching">Caching</h3><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=RAID&veaction=edit&section=8" title="Change section: Caching" class="mw-editsection-visualeditor"><span>change</span></a><span class="mw-editsection-divider"> | </span><a href="/w/index.php?title=RAID&action=edit&section=8" title="Edit section's source code: Caching"><span>change source</span></a><span class="mw-editsection-bracket">]</span></span></div> <p><a href="/wiki/Cache" class="mw-redirect" title="Cache">Caching</a> is a technology that also has its uses in RAID systems. There are different kinds of caches that are used in RAID systems: </p> <ul><li><a href="/wiki/Operating_system" title="Operating system">Operating system</a></li> <li>RAID controller</li> <li>Enterprise disk array</li></ul> <p>In modern systems, a write request is shown as done when the data has been written to the cache. This does not mean that the data has been written to the disk. Requests from the cache are not necessarily handled in the same order that they were written to the cache. This makes it possible that, if the system fails, sometimes some data has not been written to the disk involved. For this reason, many systems have a cache that is backed by a <a href="/wiki/Battery" title="Battery">battery</a>. </p> <div class="mw-heading mw-heading3"><h3 id="Mirroring:_More_than_one_copy_of_the_data">Mirroring: More than one copy of the data</h3><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=RAID&veaction=edit&section=9" title="Change section: Mirroring: More than one copy of the data" class="mw-editsection-visualeditor"><span>change</span></a><span class="mw-editsection-divider"> | </span><a href="/w/index.php?title=RAID&action=edit&section=9" title="Edit section's source code: Mirroring: More than one copy of the data"><span>change source</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>When talking about a <a href="/wiki/Mirror" title="Mirror">mirror</a>, this is a very simple idea. Instead of the data being in only one place, there are several copies of the data. These copies usually are on different hard disks (or disk partitions). If there are two copies, one of them can fail without the data being affected (as it still is on the other copy). Mirroring can also give a boost when reading data. It will always be taken from the fastest disk that responds. Writing data is slower though, because all disks need to be updated. </p> <div class="mw-heading mw-heading3"><h3 id="Striping:_Part_of_the_data_is_on_another_disk">Striping: Part of the data is on another disk</h3><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=RAID&veaction=edit&section=10" title="Change section: Striping: Part of the data is on another disk" class="mw-editsection-visualeditor"><span>change</span></a><span class="mw-editsection-divider"> | </span><a href="/w/index.php?title=RAID&action=edit&section=10" title="Edit section's source code: Striping: Part of the data is on another disk"><span>change source</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>With striping, the data is split into different parts. These parts then end up on different disks (or disk partitions). This means that writing data is faster, as it can be done in parallel. This does not mean that there will not be faults, as each block of data is only found on one disk. </p> <div class="mw-heading mw-heading3"><h3 id="Error_correction_and_faults">Error correction and faults</h3><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=RAID&veaction=edit&section=11" title="Change section: Error correction and faults" class="mw-editsection-visualeditor"><span>change</span></a><span class="mw-editsection-divider"> | </span><a href="/w/index.php?title=RAID&action=edit&section=11" title="Edit section's source code: Error correction and faults"><span>change source</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>It is possible to calculate different kinds of <a href="/wiki/Checksum" title="Checksum">checksums</a>. Some methods of calculating checksums allow finding a mistake. Most RAID levels that use redundancy can do this. Some methods are more difficult to do, but they allow to not only detect the error, but to fix it. </p> <div class="mw-heading mw-heading3"><h3 id="Hot_spares:_using_more_disks_than_needed">Hot spares: using more disks than needed</h3><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=RAID&veaction=edit&section=12" title="Change section: Hot spares: using more disks than needed" class="mw-editsection-visualeditor"><span>change</span></a><span class="mw-editsection-divider"> | </span><a href="/w/index.php?title=RAID&action=edit&section=12" title="Edit section's source code: Hot spares: using more disks than needed"><span>change source</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>Many of the ways to have RAID support something is called a <i>hot spare</i>. A hot spare is an empty disk that is not used in normal operation. When a disk fails, data can directly be copied onto the hot spare disk. That way, the failed disk needs to be replaced by a new empty drive to become the hot spare. </p> <div class="mw-heading mw-heading3"><h3 id="Stripe_size_and_chunk_size:_spreading_the_data_over_several_disks">Stripe size and chunk size: spreading the data over several disks</h3><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=RAID&veaction=edit&section=13" title="Change section: Stripe size and chunk size: spreading the data over several disks" class="mw-editsection-visualeditor"><span>change</span></a><span class="mw-editsection-divider"> | </span><a href="/w/index.php?title=RAID&action=edit&section=13" title="Edit section's source code: Stripe size and chunk size: spreading the data over several disks"><span>change source</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>RAID works by spreading the data over several disks. Two of the terms often used in this <a href="/wiki/Context_(language)" title="Context (language)">context</a> are <i>stripe size</i> and <i>chunk size</i>. </p><p>The <i>chunk size</i> is the smallest data block that is written to a single disk of the array. The <i>stripe size</i> is the size of a block of data that will be spread over all disks. That way, with four disks, and a stripe size of 64 <a href="/wiki/Kilobyte" title="Kilobyte">kilobytes</a> (kB), 16 kB will be written to each disk. The chunk size in this example is therefore 16 kB. Making the stripe size bigger will mean a faster data transfer rate, but also a bigger maximum <a href="/wiki/Latency" title="Latency">latency</a>. In this case, this is the time needed to get a block of data. </p> <div class="mw-heading mw-heading3"><h3 id="Putting_disk_together:_JBOD,_concatenation_or_spanning"><span id="Putting_disk_together:_JBOD.2C_concatenation_or_spanning"></span>Putting disk together: JBOD, concatenation or spanning</h3><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=RAID&veaction=edit&section=14" title="Change section: Putting disk together: JBOD, concatenation or spanning" class="mw-editsection-visualeditor"><span>change</span></a><span class="mw-editsection-divider"> | </span><a href="/w/index.php?title=RAID&action=edit&section=14" title="Edit section's source code: Putting disk together: JBOD, concatenation or spanning"><span>change source</span></a><span class="mw-editsection-bracket">]</span></span></div> <figure typeof="mw:File/Thumb"><a href="/wiki/File:JBOD.svg" class="mw-file-description"><img src="//upload.wikimedia.org/wikipedia/commons/thumb/e/e2/JBOD.svg/200px-JBOD.svg.png" decoding="async" width="200" height="200" class="mw-file-element" srcset="//upload.wikimedia.org/wikipedia/commons/thumb/e/e2/JBOD.svg/300px-JBOD.svg.png 1.5x, //upload.wikimedia.org/wikipedia/commons/thumb/e/e2/JBOD.svg/400px-JBOD.svg.png 2x" data-file-width="500" data-file-height="500" /></a><figcaption>JBOD with 3 disks of different sizes</figcaption></figure> <p>Many controllers (and also software) can put disks together in the following way: Take the first disk, till it ends, then they take the second, and so on. In that way, several smaller disks look like a larger one. This is not really RAID, as there is no redundancy. Also, spanning can combine disks where RAID 0 cannot do anything. Generally, this is called <i>just a bunch of disks</i> (JBOD). </p><p>This is like a distant relative of RAID because the logical drive is made of different physical drives. Concatenation is sometimes used to turn several small drives into one larger useful drive. This can not be done with RAID 0. For example, JBOD could combine 3 GB, 15 GB, 5.5 GB, and 12 GB drives into a logical drive at 35.5 GB, which is often more useful than the drives alone. </p><p>In the diagram to the right, data are concatenated from the end of disk 0 (block A63) to the beginning of disk 1 (block A64); end of disk 1 (block A91) to the beginning of disk 2 (block A92). If RAID 0 were used, then disk 0 and disk 2 would be truncated to 28 blocks, the size of the smallest disk in the array (disk 1) for a total size of 84 blocks. </p><p>Some RAID controllers use JBOD to talk about working on drives without RAID features. Each drive shows up separately in the operating system. This JBOD is not the same as concatenation. </p><p>Many <a href="/wiki/Linux" title="Linux">Linux</a> systems use the terms "linear mode" or "append mode". The <a href="/wiki/Mac_OS_X" class="mw-redirect" title="Mac OS X">Mac OS X</a> 10.4 implementation — called a "Concatenated Disk Set" — does not leave the user with any usable data on the remaining drives if one drive fails in a concatenated disk set, although the disks otherwise operate as described above. </p><p>Concatenation is one of the uses of the <a href="/wiki/Logical_volume_management" title="Logical volume management">Logical Volume Manager</a> in Linux. It can be used to create <a href="/w/index.php?title=Virtual&action=edit&redlink=1" class="new" title="Virtual (not yet started)">virtual</a> drives. </p> <div class="mw-heading mw-heading3"><h3 id="Drive_Clone">Drive Clone</h3><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=RAID&veaction=edit&section=15" title="Change section: Drive Clone" class="mw-editsection-visualeditor"><span>change</span></a><span class="mw-editsection-divider"> | </span><a href="/w/index.php?title=RAID&action=edit&section=15" title="Edit section's source code: Drive Clone"><span>change source</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>Most modern hard disks have a standard called <a href="/wiki/Self-Monitoring,_Analysis_and_Reporting_Technology" class="mw-redirect" title="Self-Monitoring, Analysis and Reporting Technology">Self-Monitoring, Analysis and Reporting Technology</a> (S.M.A.R.T). SMART allows to monitor certain things on a hard disk drive. Certain controllers allow to replace a single hard disk even before it fails, for example because S.M.A.R.T or another disk test reports too many correctable errors. To do this, the controller will copy all the data onto a hot spare drive. After this, the disk can be replaced by another (which will simply become the new hot spare). </p> <div class="mw-heading mw-heading3"><h3 id="Different_setups">Different setups</h3><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=RAID&veaction=edit&section=16" title="Change section: Different setups" class="mw-editsection-visualeditor"><span>change</span></a><span class="mw-editsection-divider"> | </span><a href="/w/index.php?title=RAID&action=edit&section=16" title="Edit section's source code: Different setups"><span>change source</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>The setup of the disks and how they use the techniques above affects the performance and reliability of the system. When more disks are used, one of the disks is more likely to fail. Because of this, mechanisms have to be built to be able to find and fix errors. This makes the whole system more reliable, as it is able to survive and repair the failure. </p><p><br /> </p> <div class="mw-heading mw-heading2"><h2 id="Basics:_simple_RAID_levels">Basics: simple RAID levels</h2><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=RAID&veaction=edit&section=17" title="Change section: Basics: simple RAID levels" class="mw-editsection-visualeditor"><span>change</span></a><span class="mw-editsection-divider"> | </span><a href="/w/index.php?title=RAID&action=edit&section=17" title="Edit section's source code: Basics: simple RAID levels"><span>change source</span></a><span class="mw-editsection-bracket">]</span></span></div> <div class="mw-heading mw-heading3"><h3 id="RAID_levels_in_common_use">RAID levels in common use</h3><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=RAID&veaction=edit&section=18" title="Change section: RAID levels in common use" class="mw-editsection-visualeditor"><span>change</span></a><span class="mw-editsection-divider"> | </span><a href="/w/index.php?title=RAID&action=edit&section=18" title="Edit section's source code: RAID levels in common use"><span>change source</span></a><span class="mw-editsection-bracket">]</span></span></div> <div class="mw-heading mw-heading4"><h4 id="RAID_0_"striping""><span id="RAID_0_.22striping.22"></span>RAID 0 "striping"</h4><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=RAID&veaction=edit&section=19" title="Change section: RAID 0 "striping"" class="mw-editsection-visualeditor"><span>change</span></a><span class="mw-editsection-divider"> | </span><a href="/w/index.php?title=RAID&action=edit&section=19" title="Edit section's source code: RAID 0 "striping""><span>change source</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>RAID 0 is not really RAID because it is not <a href="/wiki/Redundancy" title="Redundancy">redundant</a>. With RAID 0, disks are simply put together to make a large disk. This is called "striping". When one disk fails, the whole array fails. Therefore, RAID 0 is rarely used for important data, but reading and writing data from the disk can be faster with striping because each disk reads part of the file at the same time. </p><p>With RAID 0, disk blocks that come after one another are usually placed on different disks. For this reason, all disks used by a RAID 0 should be the same size. </p><p>RAID 0 is often used for <a href="/wiki/Swapspace" class="mw-redirect" title="Swapspace">Swapspace</a> on <a href="/wiki/Linux" title="Linux">Linux</a> or <a href="/wiki/Unix" class="mw-redirect" title="Unix">Unix</a>-like operating systems. </p> <div class="mw-heading mw-heading4"><h4 id="RAID_1_"mirroring""><span id="RAID_1_.22mirroring.22"></span>RAID 1 "mirroring"</h4><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=RAID&veaction=edit&section=20" title="Change section: RAID 1 "mirroring"" class="mw-editsection-visualeditor"><span>change</span></a><span class="mw-editsection-divider"> | </span><a href="/w/index.php?title=RAID&action=edit&section=20" title="Edit section's source code: RAID 1 "mirroring""><span>change source</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>With RAID 1, two disks are put together. Both hold the same data, one is "mirroring" the other. This is easy, fast configuration whether implemented with a hardware controller or by software. </p> <div class="mw-heading mw-heading4"><h4 id="RAID_5_"striping_with_distributed_parity""><span id="RAID_5_.22striping_with_distributed_parity.22"></span>RAID 5 "striping with distributed parity"</h4><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=RAID&veaction=edit&section=21" title="Change section: RAID 5 "striping with distributed parity"" class="mw-editsection-visualeditor"><span>change</span></a><span class="mw-editsection-divider"> | </span><a href="/w/index.php?title=RAID&action=edit&section=21" title="Edit section's source code: RAID 5 "striping with distributed parity""><span>change source</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>RAID Level 5 is what is probably used most of the time. At least three hard disks are needed to build a RAID 5 storage array. Each block of data will be stored in three different places. Two of these places will store the block as it is, the third will store a checksum. This checksum is a special case of a <a href="/wiki/Reed-Solomon_error_correction" class="mw-redirect" title="Reed-Solomon error correction">Reed-Solomon code</a> that only uses bitwise addition. Usually, it is calculated using the <a href="/wiki/Exclusive_disjunction" title="Exclusive disjunction">XOR</a> method. Since this method is <a href="/wiki/Symmetry" title="Symmetry">symmetric</a>, one lost data block can be rebuilt from the other data block and the checksum. For each block, a different disk will hold the parity block which holds the checksum. This is done to increase redundancy. Any disk can fail. Overall, there will be one disk holding the checksums, so the total usable capacity will be that of all disks except for one. The size of the resulting logical disk will be the size of all disks together, except for one disk which holds parity information. </p><p>Of course this is slower than RAID level 1, since on every write, all disks need to be read to calculate and update the parity information. The read performance of RAID 5 is almost as good as RAID 0 for the same number of disks. Except for the parity blocks, the distribution of data over the drives follows the same pattern as RAID 0. The reason RAID 5 is slightly slower is that the disks must skip over the parity blocks. </p><p>A RAID 5 with a failed disk will continue to work. It is in <i>degraded mode</i>. A degraded RAID 5 can be very slow. For this reason an additional disk is often added. This is called <i>hot spare</i> disk. If a disk fails, the data can be directly rebuilt onto the extra disk. RAID 5 can also be done in software quite easily. </p><p>Mainly because of performance problems of failed RAID 5 arrays, some database experts have formed a group called BAARF—the <i>Battle Against Any Raid Five</i>.<sup id="cite_ref-3" class="reference"><a href="#cite_note-3"><span class="cite-bracket">[</span>3<span class="cite-bracket">]</span></a></sup> </p><p>If the system fails while there are active writes, the parity of a stripe may become <a href="https://simple.wiktionary.org/wiki/inconsistent" class="extiw" title="wikt:inconsistent">inconsistent</a> with the data. If this is not repaired before a disk or block fails, data loss may occur. An incorrect parity will be used to reconstruct the missing block in that stripe. This problem is sometimes known as the "write hole". Battery-backed <a href="/wiki/Cache" class="mw-redirect" title="Cache">caches</a> and similar techniques are commonly used to reduce the chance for this to occur. </p> <div class="mw-heading mw-heading4"><h4 id="Pictures">Pictures</h4><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=RAID&veaction=edit&section=22" title="Change section: Pictures" class="mw-editsection-visualeditor"><span>change</span></a><span class="mw-editsection-divider"> | </span><a href="/w/index.php?title=RAID&action=edit&section=22" title="Edit section's source code: Pictures"><span>change source</span></a><span class="mw-editsection-bracket">]</span></span></div> <ul class="gallery mw-gallery-traditional"> <li class="gallerybox" style="width: 155px"> <div class="thumb" style="width: 150px; height: 150px;"><span typeof="mw:File"><a href="/wiki/File:RAID_0.png" class="mw-file-description" title="RAID 0 simply puts the different blocks on the different disks. There is no redundancy."><img alt="RAID 0 simply puts the different blocks on the different disks. There is no redundancy." src="//upload.wikimedia.org/wikipedia/commons/thumb/6/63/RAID_0.png/76px-RAID_0.png" decoding="async" width="76" height="120" class="mw-file-element" srcset="//upload.wikimedia.org/wikipedia/commons/thumb/6/63/RAID_0.png/114px-RAID_0.png 1.5x, //upload.wikimedia.org/wikipedia/commons/thumb/6/63/RAID_0.png/152px-RAID_0.png 2x" data-file-width="268" data-file-height="423" /></a></span></div> <div class="gallerytext">RAID 0 simply puts the different blocks on the different disks. There is no redundancy.</div> </li> <li class="gallerybox" style="width: 155px"> <div class="thumb" style="width: 150px; height: 150px;"><span typeof="mw:File"><a href="/wiki/File:RAID_1.png" class="mw-file-description" title="With Raid 1 every block is there on both disks"><img alt="With Raid 1 every block is there on both disks" src="//upload.wikimedia.org/wikipedia/commons/thumb/a/a1/RAID_1.png/76px-RAID_1.png" decoding="async" width="76" height="120" class="mw-file-element" srcset="//upload.wikimedia.org/wikipedia/commons/thumb/a/a1/RAID_1.png/113px-RAID_1.png 1.5x, //upload.wikimedia.org/wikipedia/commons/thumb/a/a1/RAID_1.png/151px-RAID_1.png 2x" data-file-width="267" data-file-height="423" /></a></span></div> <div class="gallerytext">With Raid 1 every block is there on both disks</div> </li> <li class="gallerybox" style="width: 155px"> <div class="thumb" style="width: 150px; height: 150px;"><span typeof="mw:File"><a href="/wiki/File:RAID_5.png" class="mw-file-description" title="RAID 5 calculates special checksums for the data. Both the blocks with the checksum and those with the data are distributed over all disks."><img alt="RAID 5 calculates special checksums for the data. Both the blocks with the checksum and those with the data are distributed over all disks." src="//upload.wikimedia.org/wikipedia/commons/thumb/1/10/RAID_5.png/120px-RAID_5.png" decoding="async" width="120" height="102" class="mw-file-element" srcset="//upload.wikimedia.org/wikipedia/commons/thumb/1/10/RAID_5.png/180px-RAID_5.png 1.5x, //upload.wikimedia.org/wikipedia/commons/thumb/1/10/RAID_5.png/240px-RAID_5.png 2x" data-file-width="498" data-file-height="423" /></a></span></div> <div class="gallerytext">RAID 5 calculates special checksums for the data. Both the blocks with the checksum and those with the data are distributed over all disks.</div> </li> </ul> <div class="mw-heading mw-heading3"><h3 id="RAID_levels_used_less">RAID levels used less</h3><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=RAID&veaction=edit&section=23" title="Change section: RAID levels used less" class="mw-editsection-visualeditor"><span>change</span></a><span class="mw-editsection-divider"> | </span><a href="/w/index.php?title=RAID&action=edit&section=23" title="Edit section's source code: RAID levels used less"><span>change source</span></a><span class="mw-editsection-bracket">]</span></span></div> <div class="mw-heading mw-heading4"><h4 id="RAID_2">RAID 2</h4><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=RAID&veaction=edit&section=24" title="Change section: RAID 2" class="mw-editsection-visualeditor"><span>change</span></a><span class="mw-editsection-divider"> | </span><a href="/w/index.php?title=RAID&action=edit&section=24" title="Edit section's source code: RAID 2"><span>change source</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>This was used with very large computers. Special expensive disks and a special controller are needed to use RAID Level 2. The data is distributed at the <a href="/wiki/Bit" class="mw-disambig" title="Bit">bit</a>-level (all other levels use <a href="/wiki/Byte" title="Byte">byte</a>-level actions). Special calculations are done. Data is split up into static sequences of bits. 8 data bits and 2 parity bits are put together. Then a <a href="/wiki/Hamming_code" title="Hamming code">Hamming code</a> is calculated. The fragments of the Hamming code are then distributed over the different disks. </p><p>RAID 2 is the only RAID level that can <i>repair</i> errors, the other RAID levels can only detect them. When they find that the information needed does not make sense, they will simply rebuild it. This is done with calculations, using information on the other disks. If that information is missing or wrong, they cannot do much. Because it uses Hamming codes, RAID 2 can find out which piece of the information is wrong, and correct only that piece. </p><p>RAID 2 needs at least 10 disks to work. Because of its complexity and its need for very expensive and special hardware, RAID 2 is no longer used very much. </p> <div class="mw-heading mw-heading4"><h4 id="RAID_3_"striping_with_dedicated_parity""><span id="RAID_3_.22striping_with_dedicated_parity.22"></span>RAID 3 "striping with dedicated parity"</h4><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=RAID&veaction=edit&section=25" title="Change section: RAID 3 "striping with dedicated parity"" class="mw-editsection-visualeditor"><span>change</span></a><span class="mw-editsection-divider"> | </span><a href="/w/index.php?title=RAID&action=edit&section=25" title="Edit section's source code: RAID 3 "striping with dedicated parity""><span>change source</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>Raid Level 3 is much like RAID Level 0. An additional disk is added to store parity information. This is done by bitwise addition of the value of a block on the other disks. The parity information is stored on a separate (dedicated) disk. This is not good, because if the parity disk crashes, the parity information is lost. </p><p>RAID Level 3 is usually done with at least 3 disks. A two-disk setup is identical to a RAID Level 0. </p> <div class="mw-heading mw-heading4"><h4 id="RAID_4_"striping_with_dedicated_parity""><span id="RAID_4_.22striping_with_dedicated_parity.22"></span>RAID 4 "striping with dedicated parity"</h4><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=RAID&veaction=edit&section=26" title="Change section: RAID 4 "striping with dedicated parity"" class="mw-editsection-visualeditor"><span>change</span></a><span class="mw-editsection-divider"> | </span><a href="/w/index.php?title=RAID&action=edit&section=26" title="Edit section's source code: RAID 4 "striping with dedicated parity""><span>change source</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>This is very similar to RAID 3, except that the parity information is calculated over larger blocks, and not single bytes. This is like RAID 5. At least three disks are needed for a RAID 4 array. </p> <div class="mw-heading mw-heading4"><h4 id="RAID_6">RAID 6</h4><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=RAID&veaction=edit&section=27" title="Change section: RAID 6" class="mw-editsection-visualeditor"><span>change</span></a><span class="mw-editsection-divider"> | </span><a href="/w/index.php?title=RAID&action=edit&section=27" title="Edit section's source code: RAID 6"><span>change source</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>RAID level 6 was not an original RAID level. It adds an additional parity block to a RAID 5 array. It needs at least four disks (two disks for the capacity, two disks for redundancy). RAID 5 can be seen as a special case of a <a href="/wiki/Reed-Solomon_error_correction" class="mw-redirect" title="Reed-Solomon error correction">Reed-Solomon code</a>. RAID 5 is a special case, though, it only needs addition in the <a href="/wiki/Galois_field" title="Galois field">Galois field</a> <i>GF(2)</i>. This is easy to do with XORs. RAID 6 extends these calculations. It is no longer a special case, and all of the calculations need to be done. With RAID 6, an extra <a href="/wiki/Checksum" title="Checksum">checksum</a> (called <a href="/wiki/Polynomial" title="Polynomial">polynomial</a>) is used, usually of <i>GF (2<sup>8</sup>)</i>. With this approach it is possible to protect against any number of failed disks. RAID 6 is for the case of using two checksums to protect against the loss of two disks. </p><p>Like with RAID 5, parity and data are on different disks for each block. The two parity blocks are also located on different disks. </p><p>There are different ways to do RAID 6. They are different in their write performance, and in how much calculations are needed. Being able to do faster writes usually means more calculations are needed. </p><p>RAID 6 is slower than RAID 5, but it allows the RAID to continue with any two disks failed. RAID 6 is becoming popular because it allows an array to be rebuilt after a single-drive failure even if one of the remaining disks has one or more bad sectors. </p> <div class="mw-heading mw-heading4"><h4 id="Pictures_2">Pictures</h4><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=RAID&veaction=edit&section=28" title="Change section: Pictures" class="mw-editsection-visualeditor"><span>change</span></a><span class="mw-editsection-divider"> | </span><a href="/w/index.php?title=RAID&action=edit&section=28" title="Edit section's source code: Pictures"><span>change source</span></a><span class="mw-editsection-bracket">]</span></span></div> <ul class="gallery mw-gallery-traditional"> <li class="gallerybox" style="width: 155px"> <div class="thumb" style="width: 150px; height: 150px;"><span typeof="mw:File"><a href="/wiki/File:RAID_3.png" class="mw-file-description" title="RAID 3 is much like RAID level 0. An extra disk is added that will hold a checksum for each block of data."><img alt="RAID 3 is much like RAID level 0. An extra disk is added that will hold a checksum for each block of data." src="//upload.wikimedia.org/wikipedia/commons/thumb/f/f5/RAID_3.png/120px-RAID_3.png" decoding="async" width="120" height="102" class="mw-file-element" srcset="//upload.wikimedia.org/wikipedia/commons/thumb/f/f5/RAID_3.png/180px-RAID_3.png 1.5x, //upload.wikimedia.org/wikipedia/commons/thumb/f/f5/RAID_3.png/240px-RAID_3.png 2x" data-file-width="504" data-file-height="427" /></a></span></div> <div class="gallerytext">RAID 3 is much like RAID level 0. An extra disk is added that will hold a checksum for each block of data.</div> </li> <li class="gallerybox" style="width: 155px"> <div class="thumb" style="width: 150px; height: 150px;"><span typeof="mw:File"><a href="/wiki/File:RAID_4.svg" class="mw-file-description" title="RAID 4 is similar to RAID level 3, but calculates parity over larger blocks of data"><img alt="RAID 4 is similar to RAID level 3, but calculates parity over larger blocks of data" src="//upload.wikimedia.org/wikipedia/commons/thumb/a/ad/RAID_4.svg/120px-RAID_4.svg.png" decoding="async" width="120" height="89" class="mw-file-element" srcset="//upload.wikimedia.org/wikipedia/commons/thumb/a/ad/RAID_4.svg/180px-RAID_4.svg.png 1.5x, //upload.wikimedia.org/wikipedia/commons/thumb/a/ad/RAID_4.svg/240px-RAID_4.svg.png 2x" data-file-width="675" data-file-height="500" /></a></span></div> <div class="gallerytext">RAID 4 is similar to RAID level 3, but calculates parity over larger blocks of data</div> </li> <li class="gallerybox" style="width: 155px"> <div class="thumb" style="width: 150px; height: 150px;"><span typeof="mw:File"><a href="/wiki/File:RAID6.png" class="mw-file-description" title="RAID 6 is similar to RAID 5, but it calculates two different checksums. This allows for two disks to fail, without data loss."><img alt="RAID 6 is similar to RAID 5, but it calculates two different checksums. This allows for two disks to fail, without data loss." src="//upload.wikimedia.org/wikipedia/commons/thumb/8/85/RAID6.png/120px-RAID6.png" decoding="async" width="120" height="72" class="mw-file-element" srcset="//upload.wikimedia.org/wikipedia/commons/thumb/8/85/RAID6.png/180px-RAID6.png 1.5x, //upload.wikimedia.org/wikipedia/commons/thumb/8/85/RAID6.png/240px-RAID6.png 2x" data-file-width="839" data-file-height="505" /></a></span></div> <div class="gallerytext">RAID 6 is similar to RAID 5, but it calculates two different checksums. This allows for two disks to fail, without data loss.</div> </li> </ul> <div class="mw-heading mw-heading3"><h3 id="Non-standard_RAID_levels">Non-standard RAID levels</h3><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=RAID&veaction=edit&section=29" title="Change section: Non-standard RAID levels" class="mw-editsection-visualeditor"><span>change</span></a><span class="mw-editsection-divider"> | </span><a href="/w/index.php?title=RAID&action=edit&section=29" title="Edit section's source code: Non-standard RAID levels"><span>change source</span></a><span class="mw-editsection-bracket">]</span></span></div> <div class="mw-heading mw-heading4"><h4 id="Double_parity_/_Diagonal_parity"><span id="Double_parity_.2F_Diagonal_parity"></span>Double parity / Diagonal parity</h4><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=RAID&veaction=edit&section=30" title="Change section: Double parity / Diagonal parity" class="mw-editsection-visualeditor"><span>change</span></a><span class="mw-editsection-divider"> | </span><a href="/w/index.php?title=RAID&action=edit&section=30" title="Edit section's source code: Double parity / Diagonal parity"><span>change source</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>RAID 6 uses two parity blocks. These are calculated in a special way over a <a href="/wiki/Polynomial" title="Polynomial">polynomial</a>. <i>Double parity</i> RAID (also called <i>diagonal parity</i> RAID)<sup id="cite_ref-4" class="reference"><a href="#cite_note-4"><span class="cite-bracket">[</span>4<span class="cite-bracket">]</span></a></sup> uses a different polynomial for each of these parity blocks. Recently, the industry association that defined RAID said that double parity RAID is a different form of RAID 6. </p> <div class="mw-heading mw-heading4"><h4 id="RAID-DP">RAID-DP</h4><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=RAID&veaction=edit&section=31" title="Change section: RAID-DP" class="mw-editsection-visualeditor"><span>change</span></a><span class="mw-editsection-divider"> | </span><a href="/w/index.php?title=RAID&action=edit&section=31" title="Edit section's source code: RAID-DP"><span>change source</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>RAID-DP is another way of having double parity. </p> <div class="mw-heading mw-heading4"><h4 id="RAID_1.5">RAID 1.5</h4><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=RAID&veaction=edit&section=32" title="Change section: RAID 1.5" class="mw-editsection-visualeditor"><span>change</span></a><span class="mw-editsection-divider"> | </span><a href="/w/index.php?title=RAID&action=edit&section=32" title="Edit section's source code: RAID 1.5"><span>change source</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>RAID 1.5 (not to be confused with RAID 15, which is different) is a proprietary RAID implementation. Like RAID 1, it only uses two disks, but it does both striping and mirroring (similar to RAID 10). Most things are done in hardware. </p> <div class="mw-heading mw-heading4"><h4 id="RAID_5E,_RAID_5EE_and_RAID_6E"><span id="RAID_5E.2C_RAID_5EE_and_RAID_6E"></span>RAID 5E, RAID 5EE and RAID 6E</h4><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=RAID&veaction=edit&section=33" title="Change section: RAID 5E, RAID 5EE and RAID 6E" class="mw-editsection-visualeditor"><span>change</span></a><span class="mw-editsection-divider"> | </span><a href="/w/index.php?title=RAID&action=edit&section=33" title="Edit section's source code: RAID 5E, RAID 5EE and RAID 6E"><span>change source</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>RAID 5E, RAID 5EE and RAID 6E (with the added <i>E</i> for <i>Enhanced</i>) generally refer to different types of RAID 5 or RAID 6 with a hot spare. With these implementations, the hot spare drive is not a physical drive. Rather, it exists in the form of free space on the disks. This increases performance, but it means that a hot spare cannot be shared between different arrays. The scheme was introduced by <a href="/wiki/IBM" title="IBM">IBM</a> ServeRAID around 2001.<sup id="cite_ref-5" class="reference"><a href="#cite_note-5"><span class="cite-bracket">[</span>5<span class="cite-bracket">]</span></a></sup> </p> <div class="mw-heading mw-heading4"><h4 id="RAID_7">RAID 7</h4><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=RAID&veaction=edit&section=34" title="Change section: RAID 7" class="mw-editsection-visualeditor"><span>change</span></a><span class="mw-editsection-divider"> | </span><a href="/w/index.php?title=RAID&action=edit&section=34" title="Edit section's source code: RAID 7"><span>change source</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>This is a proprietary implementation. It adds <a href="/wiki/Cache" class="mw-redirect" title="Cache">caching</a> to a RAID 3 or RAID 4 array.<sup id="cite_ref-6" class="reference"><a href="#cite_note-6"><span class="cite-bracket">[</span>6<span class="cite-bracket">]</span></a></sup> </p> <div class="mw-heading mw-heading4"><h4 id="Intel_Matrix_RAID">Intel Matrix RAID</h4><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=RAID&veaction=edit&section=35" title="Change section: Intel Matrix RAID" class="mw-editsection-visualeditor"><span>change</span></a><span class="mw-editsection-divider"> | </span><a href="/w/index.php?title=RAID&action=edit&section=35" title="Edit section's source code: Intel Matrix RAID"><span>change source</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>Some Intel main boards have RAID chip that have this feature. It uses two or three disks, and then partitions them equally to form a combination of RAID 0, RAID 1, RAID 5 or RAID 1+0 levels. </p> <div class="mw-heading mw-heading4"><h4 id="Linux_MD_RAID_driver">Linux MD RAID driver</h4><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=RAID&veaction=edit&section=36" title="Change section: Linux MD RAID driver" class="mw-editsection-visualeditor"><span>change</span></a><span class="mw-editsection-divider"> | </span><a href="/w/index.php?title=RAID&action=edit&section=36" title="Edit section's source code: Linux MD RAID driver"><span>change source</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>This is the name for the driver that allows to do software RAID with <a href="/wiki/Linux" title="Linux">Linux</a>. In addition to the normal RAID levels 0-6, it also has a RAID 10 implementation. Since Kernel 2.6.9, RAID 10 is a single level.<sup id="cite_ref-7" class="reference"><a href="#cite_note-7"><span class="cite-bracket">[</span>7<span class="cite-bracket">]</span></a></sup> The implementation has some non-standard features.<sup id="cite_ref-8" class="reference"><a href="#cite_note-8"><span class="cite-bracket">[</span>8<span class="cite-bracket">]</span></a></sup> </p> <div class="mw-heading mw-heading4"><h4 id="RAID_Z">RAID Z</h4><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=RAID&veaction=edit&section=37" title="Change section: RAID Z" class="mw-editsection-visualeditor"><span>change</span></a><span class="mw-editsection-divider"> | </span><a href="/w/index.php?title=RAID&action=edit&section=37" title="Edit section's source code: RAID Z"><span>change source</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>Sun has implemented a <a href="/wiki/File_system" title="File system">file system</a> called <a href="/wiki/ZFS" title="ZFS">ZFS</a>. This file system is optimised for handling large amounts of data. It includes a <a href="/wiki/Logical_volume_management" title="Logical volume management">Logical Volume Manager</a>. It also includes a feature called <b>RAID-Z</b>. It avoids the problem called <i><a href="#Atomicity">RAID 5 write hole</a></i> <sup id="cite_ref-9" class="reference"><a href="#cite_note-9"><span class="cite-bracket">[</span>9<span class="cite-bracket">]</span></a></sup> because it has a <a href="/wiki/Copy-on-write" title="Copy-on-write">copy-on-write</a> policy: It does not overwrite the data directly, but writes new data in a new location on the disk. When the write was successful, the old data is deleted. It avoids the need for read-modify-write operations for small writes, because it only writes full-stripes. Small blocks are mirrored instead of parity protected, which is possible because the file system knows the way the storage is organised. It can therefore allocate extra space if necessary. There is also <i>RAID-Z2</i> which uses two forms of parity to achieve results similar to RAID 6: the ability to survive up to two drive failures without losing data.<sup id="cite_ref-10" class="reference"><a href="#cite_note-10"><span class="cite-bracket">[</span>10<span class="cite-bracket">]</span></a></sup> </p> <div class="mw-heading mw-heading4"><h4 id="Pictures_3">Pictures</h4><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=RAID&veaction=edit&section=38" title="Change section: Pictures" class="mw-editsection-visualeditor"><span>change</span></a><span class="mw-editsection-divider"> | </span><a href="/w/index.php?title=RAID&action=edit&section=38" title="Edit section's source code: Pictures"><span>change source</span></a><span class="mw-editsection-bracket">]</span></span></div> <ul class="gallery mw-gallery-traditional"> <li class="gallerybox" style="width: 155px"> <div class="thumb" style="width: 150px; height: 150px;"><span typeof="mw:File"><a href="/wiki/File:RAID_DP.png" class="mw-file-description" title="Diagram of a RAID DP (Double Parity) setup."><img alt="Diagram of a RAID DP (Double Parity) setup." src="//upload.wikimedia.org/wikipedia/commons/thumb/9/98/RAID_DP.png/120px-RAID_DP.png" decoding="async" width="120" height="83" class="mw-file-element" srcset="//upload.wikimedia.org/wikipedia/commons/thumb/9/98/RAID_DP.png/180px-RAID_DP.png 1.5x, //upload.wikimedia.org/wikipedia/commons/thumb/9/98/RAID_DP.png/240px-RAID_DP.png 2x" data-file-width="614" data-file-height="423" /></a></span></div> <div class="gallerytext">Diagram of a RAID DP (Double Parity) setup.</div> </li> <li class="gallerybox" style="width: 155px"> <div class="thumb" style="width: 150px; height: 150px;"><span typeof="mw:File"><a href="/wiki/File:RAID_MATRIX.png" class="mw-file-description" title="A Matrix RAID setup."><img alt="A Matrix RAID setup." src="//upload.wikimedia.org/wikipedia/commons/thumb/1/1c/RAID_MATRIX.png/99px-RAID_MATRIX.png" decoding="async" width="99" height="120" class="mw-file-element" srcset="//upload.wikimedia.org/wikipedia/commons/thumb/1/1c/RAID_MATRIX.png/148px-RAID_MATRIX.png 1.5x, //upload.wikimedia.org/wikipedia/commons/thumb/1/1c/RAID_MATRIX.png/197px-RAID_MATRIX.png 2x" data-file-width="441" data-file-height="536" /></a></span></div> <div class="gallerytext">A Matrix RAID setup.</div> </li> </ul> <div class="mw-heading mw-heading2"><h2 id="Joining_RAID_levels">Joining RAID levels</h2><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=RAID&veaction=edit&section=39" title="Change section: Joining RAID levels" class="mw-editsection-visualeditor"><span>change</span></a><span class="mw-editsection-divider"> | </span><a href="/w/index.php?title=RAID&action=edit&section=39" title="Edit section's source code: Joining RAID levels"><span>change source</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>With RAID different disks can be put together to get a logical disk.The user will only see the logical disk. Each one of the RAID levels mentioned above has good and bad points. But RAID can also work with logical disks. That way one of the RAID levels above can be used with a set of logical disks. Many people note it by writing the numbers together. Sometimes, they write a '+' or an '&' in between. Common combinations (using two levels) are the following: </p> <ul><li><i>RAID 0+1</i>: Two or more RAID 0 arrays are combined to a RAID 1 array; This is called a <i>Mirror of stripes</i></li> <li><i>RAID 1+0</i>: Same as RAID 0+1, but RAID levels reversed; <i>Stripe of Mirrors</i>. This makes disk failure rarer than RAID 0+1 above.</li> <li><i>RAID 5+0</i>: Stripe several RAID 5's with a RAID 0. One disk of each RAID 5 can fail, but makes that RAID 5 the single point of failure; if another disk of that array fails, all the data of the array will be lost.</li> <li><i>RAID 5+1</i>: Mirror a set of RAID 5: In a situations where the RAID is made of six disks, any three can fail (without data being lost).</li> <li><i>RAID 6+0</i>: Stripe several RAID 6 arrays over a RAID 0; Two disks of each RAID 6 can fail without data loss.</li></ul> <p>With six disks of 300 GB each, a total capacity of 1.8TB, it is possible to make a RAID 5, with 1.5 TB usable space. In that array, one disk can fail without data loss. With RAID 50, the space is reduced to 1.2 TB, but one disk of each RAID 5 can fail, plus there is a noticeable increase in performance. RAID 51 reduces the usable size to 900 GB, but allows any three drives to fail. </p> <ul class="gallery mw-gallery-traditional"> <li class="gallerybox" style="width: 155px"> <div class="thumb" style="width: 150px; height: 150px;"><span typeof="mw:File"><a href="/wiki/File:RAID_0%2B1.png" class="mw-file-description" title="RAID 0+1: Several RAID 0 arrays are combined with a RAID 1"><img alt="RAID 0+1: Several RAID 0 arrays are combined with a RAID 1" src="//upload.wikimedia.org/wikipedia/commons/thumb/4/4e/RAID_0%2B1.png/112px-RAID_0%2B1.png" decoding="async" width="112" height="120" class="mw-file-element" srcset="//upload.wikimedia.org/wikipedia/commons/thumb/4/4e/RAID_0%2B1.png/168px-RAID_0%2B1.png 1.5x, //upload.wikimedia.org/wikipedia/commons/thumb/4/4e/RAID_0%2B1.png/224px-RAID_0%2B1.png 2x" data-file-width="537" data-file-height="575" /></a></span></div> <div class="gallerytext">RAID 0+1: Several RAID 0 arrays are combined with a RAID 1</div> </li> <li class="gallerybox" style="width: 155px"> <div class="thumb" style="width: 150px; height: 150px;"><span typeof="mw:File"><a href="/wiki/File:RAID_10.svg" class="mw-file-description" title="RAID 1+0: More robust than RAID 0+1; supports multiple drive failures, as long as no two drives that make a mirror fail."><img alt="RAID 1+0: More robust than RAID 0+1; supports multiple drive failures, as long as no two drives that make a mirror fail." src="//upload.wikimedia.org/wikipedia/commons/thumb/b/bb/RAID_10.svg/120px-RAID_10.svg.png" decoding="async" width="120" height="120" class="mw-file-element" srcset="//upload.wikimedia.org/wikipedia/commons/thumb/b/bb/RAID_10.svg/180px-RAID_10.svg.png 1.5x, //upload.wikimedia.org/wikipedia/commons/thumb/b/bb/RAID_10.svg/240px-RAID_10.svg.png 2x" data-file-width="1000" data-file-height="1000" /></a></span></div> <div class="gallerytext">RAID 1+0: More robust than RAID 0+1; supports multiple drive failures, as long as no two drives that make a mirror fail.</div> </li> <li class="gallerybox" style="width: 155px"> <div class="thumb" style="width: 150px; height: 150px;"><span typeof="mw:File"><a href="/wiki/File:RAID_51.png" class="mw-file-description" title="RAID 5+1: Any three drives of this can fail, without data loss."><img alt="RAID 5+1: Any three drives of this can fail, without data loss." src="//upload.wikimedia.org/wikipedia/commons/thumb/0/06/RAID_51.png/120px-RAID_51.png" decoding="async" width="120" height="90" class="mw-file-element" srcset="//upload.wikimedia.org/wikipedia/commons/thumb/0/06/RAID_51.png/180px-RAID_51.png 1.5x, //upload.wikimedia.org/wikipedia/commons/thumb/0/06/RAID_51.png/240px-RAID_51.png 2x" data-file-width="767" data-file-height="575" /></a></span></div> <div class="gallerytext">RAID 5+1: Any three drives of this can fail, without data loss.</div> </li> </ul> <div class="mw-heading mw-heading2"><h2 id="Making_a_RAID">Making a RAID</h2><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=RAID&veaction=edit&section=40" title="Change section: Making a RAID" class="mw-editsection-visualeditor"><span>change</span></a><span class="mw-editsection-divider"> | </span><a href="/w/index.php?title=RAID&action=edit&section=40" title="Edit section's source code: Making a RAID"><span>change source</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>There are different ways to make a RAID. It can either be done with <a href="/wiki/Software" title="Software">software</a>, or with <a href="/wiki/Computer_hardware" title="Computer hardware">hardware</a>. </p> <div class="mw-heading mw-heading3"><h3 id="Software_RAID">Software RAID</h3><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=RAID&veaction=edit&section=41" title="Change section: Software RAID" class="mw-editsection-visualeditor"><span>change</span></a><span class="mw-editsection-divider"> | </span><a href="/w/index.php?title=RAID&action=edit&section=41" title="Edit section's source code: Software RAID"><span>change source</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>A RAID can be made with software in two different ways. In the case of Software RAID, the disks are connected like normal hard disks. It is the computer that makes the RAID work. This means that for each access the <a href="/wiki/CPU" class="mw-redirect" title="CPU">CPU</a> also needs to do the calculations for the RAID. The calculations for RAID 0 or RAID 1 are simple. However, the calculations for RAID 5, RAID 6, or one of the combined RAID levels can be a lot of work. In a software RAID, automatically booting from an array that failed may be a difficult thing to do. Finally, the way RAID is done in software depends on the <a href="/wiki/Operating_system" title="Operating system">operating system</a> used; it is generally not possible to re-build a Software RAID array with a different operating system. Operating systems usually use hard disk partitions rather than whole hard disks to make RAID arrays. </p> <div class="mw-heading mw-heading3"><h3 id="Hardware_RAID">Hardware RAID</h3><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=RAID&veaction=edit&section=42" title="Change section: Hardware RAID" class="mw-editsection-visualeditor"><span>change</span></a><span class="mw-editsection-divider"> | </span><a href="/w/index.php?title=RAID&action=edit&section=42" title="Edit section's source code: Hardware RAID"><span>change source</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>A RAID can also be made with hardware. In this case, a special disk controller is used; this controller card hides the fact that it is doing RAID from the <a href="/wiki/Operating_system" title="Operating system">operating system</a> and the user. The calculations of checksum information, and other RAID-related calculations are done on a special microchip in that controller. This makes the RAID independent of the operating system. The operating system will not see the RAID, it will see a single disk. Different manufacturers do RAID in different ways. This means that a RAID built with one hardware RAID controller cannot be rebuilt by another RAID controller of a different manufacturer. Hardware RAID controllers are often expensive to buy. </p> <div class="mw-heading mw-heading3"><h3 id="Hardware-assisted_RAID">Hardware-assisted RAID</h3><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=RAID&veaction=edit&section=43" title="Change section: Hardware-assisted RAID" class="mw-editsection-visualeditor"><span>change</span></a><span class="mw-editsection-divider"> | </span><a href="/w/index.php?title=RAID&action=edit&section=43" title="Edit section's source code: Hardware-assisted RAID"><span>change source</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>This is a mix between hardware RAID and software RAID. Hardware-assisted RAID uses a special controller chip (like hardware RAID), but this chip can not do many operations. It is only active when the system is started; as soon as the <a href="/wiki/Operating_system" title="Operating system">operating system</a> is fully loaded, this configuration is like software RAID. Some <a href="/wiki/Motherboard" title="Motherboard">motherboards</a> have RAID functions for the disks attached; most often, these RAID functions are done as <i>hardware-assisted RAID</i>. This means that special software is needed to be able to use these RAID functions and to be able to recover from a failed disk. </p> <div class="mw-heading mw-heading2"><h2 id="Different_terms_related_to_hardware_failures">Different terms related to hardware failures</h2><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=RAID&veaction=edit&section=44" title="Change section: Different terms related to hardware failures" class="mw-editsection-visualeditor"><span>change</span></a><span class="mw-editsection-divider"> | </span><a href="/w/index.php?title=RAID&action=edit&section=44" title="Edit section's source code: Different terms related to hardware failures"><span>change source</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>There are different terms that are used when talking about hardware failures: </p> <div class="mw-heading mw-heading3"><h3 id="Failure_rate">Failure rate</h3><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=RAID&veaction=edit&section=45" title="Change section: Failure rate" class="mw-editsection-visualeditor"><span>change</span></a><span class="mw-editsection-divider"> | </span><a href="/w/index.php?title=RAID&action=edit&section=45" title="Edit section's source code: Failure rate"><span>change source</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>The <a href="/wiki/Failure_rate" title="Failure rate">failure rate</a> is how often a system fails. The <i><a href="/wiki/Average" title="Average">mean</a> time to failure</i> (MTTF) or <i>mean time between failures</i> (MTBF) of a RAID system is the same as that of its components. A RAID system cannot protect against failures of its individual hard drives, after all. The more complicated types of RAID (anything beyond "striping" or "concatenation") can help keep the data intact even if an individual hard drive fails, though. </p> <div class="mw-heading mw-heading3"><h3 id="Mean_time_to_data_loss">Mean time to data loss</h3><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=RAID&veaction=edit&section=46" title="Change section: Mean time to data loss" class="mw-editsection-visualeditor"><span>change</span></a><span class="mw-editsection-divider"> | </span><a href="/w/index.php?title=RAID&action=edit&section=46" title="Edit section's source code: Mean time to data loss"><span>change source</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>The <i>mean time to data loss</i> (MTTDL) gives the average time before a loss of data happens in a given array.<sup id="cite_ref-11" class="reference"><a href="#cite_note-11"><span class="cite-bracket">[</span>11<span class="cite-bracket">]</span></a></sup> Mean time to data loss of a given RAID may be higher or lower than that of its hard disks. This depends on the type of RAID used. </p> <div class="mw-heading mw-heading3"><h3 id="Mean_time_to_recovery">Mean time to recovery</h3><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=RAID&veaction=edit&section=47" title="Change section: Mean time to recovery" class="mw-editsection-visualeditor"><span>change</span></a><span class="mw-editsection-divider"> | </span><a href="/w/index.php?title=RAID&action=edit&section=47" title="Edit section's source code: Mean time to recovery"><span>change source</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>Arrays that have <a href="/wiki/Redundancy" title="Redundancy">redundancy</a> can recover from some failures. The <i>mean time to recovery</i> shows how long it takes until a failed array is back to its normal state. This adds both the time to replace a failed disk mechanism as well as time to re-build the array (i.e. to replicate data for redundancy). </p> <div class="mw-heading mw-heading3"><h3 id="Unrecoverable_bit_error_rate">Unrecoverable bit error rate</h3><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=RAID&veaction=edit&section=48" title="Change section: Unrecoverable bit error rate" class="mw-editsection-visualeditor"><span>change</span></a><span class="mw-editsection-divider"> | </span><a href="/w/index.php?title=RAID&action=edit&section=48" title="Edit section's source code: Unrecoverable bit error rate"><span>change source</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>The <i>unrecoverable bit error rate</i> (UBE) tells how long a disk drive will be unable to recover data after using <a href="/wiki/Cyclic_redundancy_check" title="Cyclic redundancy check">cyclic redundancy check</a> (CRC) codes and multiple retries. </p> <div class="mw-heading mw-heading2"><h2 id="Problems_with_RAID">Problems with RAID</h2><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=RAID&veaction=edit&section=49" title="Change section: Problems with RAID" class="mw-editsection-visualeditor"><span>change</span></a><span class="mw-editsection-divider"> | </span><a href="/w/index.php?title=RAID&action=edit&section=49" title="Edit section's source code: Problems with RAID"><span>change source</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>There are also certain problems with the ideas or the technology behind RAID: </p> <div class="mw-heading mw-heading3"><h3 id="Adding_disks_at_a_later_time">Adding disks at a later time</h3><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=RAID&veaction=edit&section=50" title="Change section: Adding disks at a later time" class="mw-editsection-visualeditor"><span>change</span></a><span class="mw-editsection-divider"> | </span><a href="/w/index.php?title=RAID&action=edit&section=50" title="Edit section's source code: Adding disks at a later time"><span>change source</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>Certain RAID levels allow to extend the array by simply adding hard disks, at a later time. Information such as parity blocks is often scattered on several disks. Adding a disk to the array means that a reorganisation becomes necessary. Such a reorganisation is like a re-build of the array, it can take a long time. When this is done, the additional space may not be available yet, because both the <a href="/wiki/File_system" title="File system">file system</a> on the array, and the <a href="/wiki/Operating_system" title="Operating system">operating system</a> need to be told about it. Some file systems do not support to be grown after they have been created. In such a case, all the data needs to be backed up, the array needs be re-created with the new layout, and the data needs to be restored onto it. </p><p>Another option to add storage is to create a new array, and to let a <a href="/wiki/Logical_volume_manager" class="mw-redirect" title="Logical volume manager">logical volume manager</a> handle the situation. This allows to grow almost any RAID system, even RAID1 (which by itself is limited to two disks). </p> <div class="mw-heading mw-heading3"><h3 id="Linked_failures">Linked failures</h3><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=RAID&veaction=edit&section=51" title="Change section: Linked failures" class="mw-editsection-visualeditor"><span>change</span></a><span class="mw-editsection-divider"> | </span><a href="/w/index.php?title=RAID&action=edit&section=51" title="Edit section's source code: Linked failures"><span>change source</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>The error correction mechanism in RAID assumes that failures of drives are independent. It is possible to calculate how often a piece of equipment can fail and to arrange the array to make data loss very improbable. </p><p>In practice, however, the drives were often bought together. They have roughly the same age, and have been used similarly (called <i>wear</i>). Many drives fail because of mechanical problems. The older a drive is, the more worn are its mechanical parts. Mechanical parts that are old are more likely to fail than those that are younger. This means that drive failures are no longer statistically independent. In practice, there is a chance that a second disk will also fail before the first has been recovered. This means that data loss can occur at significant rates, in practice.<sup id="cite_ref-schroeder_12-0" class="reference"><a href="#cite_note-schroeder-12"><span class="cite-bracket">[</span>12<span class="cite-bracket">]</span></a></sup> </p> <div class="mw-heading mw-heading3"><h3 id="Atomicity">Atomicity</h3><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=RAID&veaction=edit&section=52" title="Change section: Atomicity" class="mw-editsection-visualeditor"><span>change</span></a><span class="mw-editsection-divider"> | </span><a href="/w/index.php?title=RAID&action=edit&section=52" title="Edit section's source code: Atomicity"><span>change source</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>Another problem that also occurs with RAID systems is that applications expect what is called <i><a href="/wiki/Atomicity" title="Atomicity">Atomicity</a></i>: Either all of the data is written, or none is. Writing the data is known as a <i>transaction</i>. </p><p>In RAID arrays, the new data is usually written in the place where the old data was. This has become known as <i>update in-place</i>. Jim Gray, a database researcher wrote a paper in 1981 where he described this problem.<sup id="cite_ref-13" class="reference"><a href="#cite_note-13"><span class="cite-bracket">[</span>13<span class="cite-bracket">]</span></a></sup> </p><p>Very few storage systems allow <i>atomic write semantics</i>. When an object is written to disk, a RAID storage device will usually be writing all copies of the object in parallel. Very often, there is only one processor responsible for writing the data. In such a case, the writes of data to the different drives will overlap. This is known as <i>overlapped write</i> or <i>staggered write</i>. An error that occurs during the process of writing may therefore leave the redundant copies in different states. What is worse, it may leave the copies in neither the old nor the new state. Logging relies on the original data being either in the old or the new state, though. This permits backing out the logical change, but few storage systems provide an atomic write semantic on a RAID disk. </p><p>Using a battery-backed write cache can solve this problem, but only in a power failure scenario. </p><p>Transactional support is not present in all hardware RAID controllers. Therefore, many operating systems include it to protect against data loss during an interrupted write. Novell Netware, starting with version 3.x, included a transaction tracking system. Microsoft introduced transaction tracking via the journaling feature in <a href="/wiki/NTFS" title="NTFS">NTFS</a>. NetApp <a href="/wiki/Write_Anywhere_File_Layout" title="Write Anywhere File Layout">WAFL</a> file system solves it by never updating the data in place, as does <a href="/wiki/ZFS" title="ZFS">ZFS</a>. </p> <div class="mw-heading mw-heading3"><h3 id="Unrecoverable_data">Unrecoverable data</h3><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=RAID&veaction=edit&section=53" title="Change section: Unrecoverable data" class="mw-editsection-visualeditor"><span>change</span></a><span class="mw-editsection-divider"> | </span><a href="/w/index.php?title=RAID&action=edit&section=53" title="Edit section's source code: Unrecoverable data"><span>change source</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>Some sectors on a hard disk may have become unreadable because of a mistake. Some RAID implementations can deal with this situation by moving the data elsewhere and marking the sector on the disk as bad. This happens at about 1 bit in 10<sup>15</sup> in enterprise-class disk drives, and 1 bit in 10<sup>14</sup> in ordinary disk drives.<sup id="cite_ref-hdfailure_14-0" class="reference"><a href="#cite_note-hdfailure-14"><span class="cite-bracket">[</span>14<span class="cite-bracket">]</span></a></sup> Disk capacities are steadily increasing. This may mean that sometimes, a RAID cannot be rebuilt, because such an error is found when the array is rebuilt after a disk failure. Certain technologies such as RAID 6 try to address this issue, but they suffer from a very high write penalty, in other words writing data becomes very slow. </p> <div class="mw-heading mw-heading3"><h3 id="Write_cache_reliability">Write cache reliability</h3><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=RAID&veaction=edit&section=54" title="Change section: Write cache reliability" class="mw-editsection-visualeditor"><span>change</span></a><span class="mw-editsection-divider"> | </span><a href="/w/index.php?title=RAID&action=edit&section=54" title="Edit section's source code: Write cache reliability"><span>change source</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>The disk system can acknowledge the write operation as soon as the data is in the <a href="/wiki/Cache" class="mw-redirect" title="Cache">cache</a>. It does not need to wait until the data has been physically written. However, any power outage can then mean a significant data loss of any data queued in such a cache. </p><p>With hardware RAID, a battery can be used to protect this cache. This often solves the problem. When the power fails, the controller can finish writing the cache when the power is back. This solution can still fail, though: the battery may have worn out, the power may have been off for too long, the disks could be moved to another controller, the controller itself could fail. Certain systems can do periodic battery checks, but these use the battery itself, and leave it in a state where it is not fully charged. </p> <div class="mw-heading mw-heading3"><h3 id="Equipment_compatibility">Equipment compatibility</h3><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=RAID&veaction=edit&section=55" title="Change section: Equipment compatibility" class="mw-editsection-visualeditor"><span>change</span></a><span class="mw-editsection-divider"> | </span><a href="/w/index.php?title=RAID&action=edit&section=55" title="Edit section's source code: Equipment compatibility"><span>change source</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>The disk formats on different RAID controllers are not necessarily compatible. Therefore, it may not be possible to read a RAID array on different hardware. Consequently, a non-disk hardware failure may require using identical hardware, or a backup, to recover the data. </p> <div class="mw-heading mw-heading2"><h2 id="What_RAID_can_and_cannot_do">What RAID can and cannot do</h2><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=RAID&veaction=edit&section=56" title="Change section: What RAID can and cannot do" class="mw-editsection-visualeditor"><span>change</span></a><span class="mw-editsection-divider"> | </span><a href="/w/index.php?title=RAID&action=edit&section=56" title="Edit section's source code: What RAID can and cannot do"><span>change source</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>This guide was taken from a thread in a RAID-related forum. This was done to help point out the advantages and disadvantages of choosing RAID. It is directed at people who want to choose RAID for either increases in performance or redundancy. It contains links to other threads in its forum containing user-generated anecdotal reviews of their RAID experiences. </p> <div class="mw-heading mw-heading3"><h3 id="What_RAID_can_do">What RAID can do</h3><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=RAID&veaction=edit&section=57" title="Change section: What RAID can do" class="mw-editsection-visualeditor"><span>change</span></a><span class="mw-editsection-divider"> | </span><a href="/w/index.php?title=RAID&action=edit&section=57" title="Edit section's source code: What RAID can do"><span>change source</span></a><span class="mw-editsection-bracket">]</span></span></div> <ul><li><i>RAID can protect uptime.</i> RAID levels 1, 0+1/10, 5 and 6 (and their variants such as 50 and 51) make up for a mechanical hard disk failure. Even after the disk failed, the data on the array can still be used. Instead of a time consuming restore from <a href="/wiki/Videotape" title="Videotape">tape</a>, <a href="/wiki/DVD" title="DVD">DVD</a> or other slow backup media, RAID allows data to be restored to a replacement disk from the other members of the array. During this restoration process, it is available to users in a degraded state. This is very important to enterprises, as downtime quickly leads to lost earning power. For home users, it can protect uptime of large media storage arrays, which would require time consuming restoration from dozens of DVD or quite a few tapes in the event of a disk failing that is not protected by redundancy.</li> <li><i>RAID can increase performance in certain applications.</i> RAID levels 0, 5 and 6 all use striping. This allows multiple <a href="/wiki/Hard_disk" title="Hard disk">spindles</a> to increase transfer rates for linear transfers. Workstation-type applications often work with large files. They greatly benefit from disk striping. Examples for such applications are those using video or audio files. This throughput is also useful in disk-to-disk backups. RAID 1 as well as other striping-based RAID levels can improve the performance for access patterns with many simultaneous random accesses, like those used by a multi-user <a href="/wiki/Database" title="Database">database</a>.</li></ul> <div class="mw-heading mw-heading3"><h3 id="What_RAID_cannot_do">What RAID cannot do</h3><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=RAID&veaction=edit&section=58" title="Change section: What RAID cannot do" class="mw-editsection-visualeditor"><span>change</span></a><span class="mw-editsection-divider"> | </span><a href="/w/index.php?title=RAID&action=edit&section=58" title="Edit section's source code: What RAID cannot do"><span>change source</span></a><span class="mw-editsection-bracket">]</span></span></div> <ul><li><i>RAID cannot protect the data on the array.</i> A RAID array has one file system. This creates a single point of failure. There are many things that can happen to this file system other than physical disk failure. RAID cannot defend against these sources of data loss. RAID will not stop a <a href="/wiki/Computer_virus" title="Computer virus">virus</a> from destroying data. RAID will not prevent corruption. RAID will not save data when a user modifies it or deletes it by accident. RAID does not protect data from hardware failure of any component besides physical disks. RAID does not protect data from natural or man made disasters such as fires and floods. To protect data, it must be <a href="/wiki/Backup" title="Backup">backed up</a> to removable media, such as <a href="/wiki/DVD" title="DVD">DVD</a>, tape, or an external hard disk. The backup must be kept in a different place. RAID alone will not prevent a disaster, when (not if) it occurs, from turning into data loss. Disasters cannot be prevented, but backups allow data loss to be prevented.</li> <li><i>RAID cannot simplify disaster recovery.</i> When running a single disk, the disk can be used by most operating systems as they come with a common <a href="/wiki/Device_driver" title="Device driver">device driver</a>. However, most RAID controllers need special drivers. Recovery tools that work with single disks on generic controllers will require special drivers to access data on RAID arrays. If these recovery tools are poorly coded and do not allow providing for additional drivers, then a RAID array will probably be inaccessible to that recovery tool.</li> <li><i>RAID cannot provide a performance boost in all applications.</i> This statement is especially true with typical desktop application users and gamers. For most desktop applications and games the buffer strategy and seek performance of the disk(s) are more important than raw throughput. Increasing raw sustained transfer rate shows little gains for such users, as most files that they access are typically very small anyway. Disk striping using RAID 0 increases linear transfer performance, not buffer and seek performance. As a result, disk striping using RAID 0 shows little to no performance gain in most desktop applications and games, although there are exceptions. For desktop users and gamers with high performance as a goal, it is better to buy a faster, bigger, and more expensive single disk than it is to run two slower/smaller drives in RAID 0. Even running the latest, greatest, and biggest drives in RAID-0 is unlikely to boost performance more than 10%, and performance may drop in some access patterns, particularly games.</li> <li><i>It is difficult to move RAID to a new system.</i> With a single disk, it is relatively easy to move the disk to a new system. It can simply be connected to the new system, if it has the same interface available. However, this is not so easy with a RAID array. There is a certain kind of <a href="/wiki/Metadata" title="Metadata">Metadata</a> that says how the RAID is set up. A RAID <a href="/wiki/BIOS" title="BIOS">BIOS</a> must be able to read this metadata so that it can successfully construct the array and make it accessible to an operating system. Since RAID controller makers use different formats for their metadata (even controllers of different families from the same manufacturer may use incompatible metadata formats) it is almost impossible to move a RAID array to a different controller. When moving a RAID array to a new system, plans should be made to move the controller as well. With the popularity of motherboard integrated RAID controllers, this is extremely difficult. Generally, it is possible to move the RAID array members and controllers together. Software RAID in Linux and Windows Server Products can also work around this limitation, but software RAID has others (mostly performance related).</li></ul> <div class="mw-heading mw-heading2"><h2 id="Example">Example</h2><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=RAID&veaction=edit&section=59" title="Change section: Example" class="mw-editsection-visualeditor"><span>change</span></a><span class="mw-editsection-divider"> | </span><a href="/w/index.php?title=RAID&action=edit&section=59" title="Edit section's source code: Example"><span>change source</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>The RAID levels used most often are RAID 0, RAID 1, and RAID 5. Suppose there is a 3 disk setup, with 3 identical disks of 1 TB each, and the probability of failure of a drive for a given timespan is 1%. </p> <table class="wikitable"> <tbody><tr> <th>RAID Level </th> <th>Usable capacity </th> <th>Probability of failure <p>given in percent </p> </th> <th>Probability of failure <p>1 in ... cases fail </p> </th></tr> <tr> <td>0 </td> <td>3 TB </td> <td>2,9701% </td> <td>34 </td></tr> <tr> <td>1 </td> <td>1 TB </td> <td>0,0001% </td> <td>1 million </td></tr> <tr> <td>5 </td> <td>2 TB </td> <td>0,0298% </td> <td>3356 </td></tr></tbody></table> <div class="mw-heading mw-heading2"><h2 id="References">References</h2><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=RAID&veaction=edit&section=60" title="Change section: References" class="mw-editsection-visualeditor"><span>change</span></a><span class="mw-editsection-divider"> | </span><a href="/w/index.php?title=RAID&action=edit&section=60" title="Edit section's source code: References"><span>change source</span></a><span class="mw-editsection-bracket">]</span></span></div> <style data-mw-deduplicate="TemplateStyles:r9724317">.mw-parser-output .reflist{margin-bottom:0.5em;list-style-type:decimal}@media screen{.mw-parser-output .reflist{font-size:90%}}.mw-parser-output .reflist .references{font-size:100%;margin-bottom:0;list-style-type:inherit}.mw-parser-output .reflist-columns-2{column-width:30em}.mw-parser-output .reflist-columns-3{column-width:25em}.mw-parser-output .reflist-columns{margin-top:0.3em}.mw-parser-output .reflist-columns ol{margin-top:0}.mw-parser-output .reflist-columns li{page-break-inside:avoid;break-inside:avoid-column}.mw-parser-output .reflist-upper-alpha{list-style-type:upper-alpha}.mw-parser-output .reflist-upper-roman{list-style-type:upper-roman}.mw-parser-output .reflist-lower-alpha{list-style-type:lower-alpha}.mw-parser-output .reflist-lower-greek{list-style-type:lower-greek}.mw-parser-output .reflist-lower-roman{list-style-type:lower-roman}</style><div class="reflist reflist-columns references-column-width reflist-columns-2"> <ol class="references"> <li id="cite_note-Patterson88-1"><span class="mw-cite-backlink">↑ <sup><a href="#cite_ref-Patterson88_1-0">1.0</a></sup> <sup><a href="#cite_ref-Patterson88_1-1">1.1</a></sup></span> <span class="reference-text"><style data-mw-deduplicate="TemplateStyles:r9724332">.mw-parser-output cite.citation{font-style:inherit;word-wrap:break-word}.mw-parser-output .citation q{quotes:"\"""\"""'""'"}.mw-parser-output .citation:target{background-color:rgba(0,127,255,0.133)}.mw-parser-output .id-lock-free.id-lock-free a{background:url("//upload.wikimedia.org/wikipedia/commons/6/65/Lock-green.svg")right 0.1em center/9px no-repeat}.mw-parser-output .id-lock-limited.id-lock-limited a,.mw-parser-output .id-lock-registration.id-lock-registration a{background:url("//upload.wikimedia.org/wikipedia/commons/d/d6/Lock-gray-alt-2.svg")right 0.1em center/9px no-repeat}.mw-parser-output .id-lock-subscription.id-lock-subscription a{background:url("//upload.wikimedia.org/wikipedia/commons/a/aa/Lock-red-alt-2.svg")right 0.1em center/9px no-repeat}.mw-parser-output .cs1-ws-icon a{background:url("//upload.wikimedia.org/wikipedia/commons/4/4c/Wikisource-logo.svg")right 0.1em center/12px no-repeat}body:not(.skin-timeless):not(.skin-minerva) .mw-parser-output .id-lock-free a,body:not(.skin-timeless):not(.skin-minerva) .mw-parser-output .id-lock-limited a,body:not(.skin-timeless):not(.skin-minerva) .mw-parser-output .id-lock-registration a,body:not(.skin-timeless):not(.skin-minerva) .mw-parser-output .id-lock-subscription a,body:not(.skin-timeless):not(.skin-minerva) .mw-parser-output .cs1-ws-icon a{background-size:contain;padding:0 1em 0 0}.mw-parser-output .cs1-code{color:inherit;background:inherit;border:none;padding:inherit}.mw-parser-output .cs1-hidden-error{display:none;color:var(--color-error,#d33)}.mw-parser-output .cs1-visible-error{color:var(--color-error,#d33)}.mw-parser-output .cs1-maint{display:none;color:#085;margin-left:0.3em}.mw-parser-output .cs1-kern-left{padding-left:0.2em}.mw-parser-output .cs1-kern-right{padding-right:0.2em}.mw-parser-output .citation .mw-selflink{font-weight:inherit}@media screen{.mw-parser-output .cs1-format{font-size:95%}html.skin-theme-clientpref-night .mw-parser-output .cs1-maint{color:#18911f}}@media screen and (prefers-color-scheme:dark){html.skin-theme-clientpref-os .mw-parser-output .cs1-maint{color:#18911f}}</style><cite class="citation web cs1"><a rel="nofollow" class="external text" href="http://www-2.cs.cmu.edu/~garth/RAIDpaper/Patterson88.pdf">"<span class="cs1-kern-left"></span>"A Case for Redundant Arrays of Inexpensive Disks" - Patterson, Gibson, Katz"</a> <span class="cs1-format">(PDF)</span>. University of California. 1988.</cite><span title="ctx_ver=Z39.88-2004&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Abook&rft.genre=unknown&rft.btitle=%22A+Case+for+Redundant+Arrays+of+Inexpensive+Disks%22+-+Patterson%2C+Gibson%2C+Katz&rft.pub=University+of+California&rft.date=1988&rft_id=http%3A%2F%2Fwww-2.cs.cmu.edu%2F~garth%2FRAIDpaper%2FPatterson88.pdf&rfr_id=info%3Asid%2Fsimple.wikipedia.org%3ARAID" class="Z3988"></span></span> </li> <li id="cite_note-chen-2"><span class="mw-cite-backlink"><a href="#cite_ref-chen_2-0">↑</a></span> <span class="reference-text"><link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r9724332" /><cite class="citation web cs1"><a rel="nofollow" class="external text" href="http://www.thomason.org/Rio/papers/chen94_1.pdf">"RAID: High-Performance, Reliable Secondary Storage"</a> <span class="cs1-format">(PDF)</span>.</cite><span title="ctx_ver=Z39.88-2004&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Abook&rft.genre=unknown&rft.btitle=RAID%3A+High-Performance%2C+Reliable+Secondary+Storage&rft_id=http%3A%2F%2Fwww.thomason.org%2FRio%2Fpapers%2Fchen94_1.pdf&rfr_id=info%3Asid%2Fsimple.wikipedia.org%3ARAID" class="Z3988"></span></span> </li> <li id="cite_note-3"><span class="mw-cite-backlink"><a href="#cite_ref-3">↑</a></span> <span class="reference-text"><link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r9724332" /><cite class="citation web cs1"><a rel="nofollow" class="external text" href="http://www.baarf.com">"BAARF - Battle Against Any Raid Five"</a><span class="reference-accessdate">. Retrieved <span class="nowrap">2008-07-11</span></span>.</cite><span title="ctx_ver=Z39.88-2004&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Abook&rft.genre=unknown&rft.btitle=BAARF+-+Battle+Against+Any+Raid+Five&rft_id=http%3A%2F%2Fwww.baarf.com&rfr_id=info%3Asid%2Fsimple.wikipedia.org%3ARAID" class="Z3988"></span></span> </li> <li id="cite_note-4"><span class="mw-cite-backlink"><a href="#cite_ref-4">↑</a></span> <span class="reference-text"><link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r9724332" /><cite class="citation web cs1"><a rel="nofollow" class="external text" href="http://media.netapp.com/documents/wp_3298.pdf">"RAID-DP™: Network Appliance™ implementation of RAID Double Parity for data protection, a high speed implementation of RAID 6"</a> <span class="cs1-format">(PDF)</span>.</cite><span title="ctx_ver=Z39.88-2004&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Abook&rft.genre=unknown&rft.btitle=RAID-DP%E2%84%A2%3A+Network+Appliance%E2%84%A2+implementation+of+RAID+Double+Parity+for+data+protection%2C+a+high+speed+implementation+of+RAID+6&rft_id=http%3A%2F%2Fmedia.netapp.com%2Fdocuments%2Fwp_3298.pdf&rfr_id=info%3Asid%2Fsimple.wikipedia.org%3ARAID" class="Z3988"></span><sup class="noprint Inline-Template"><span style="white-space: nowrap;">[<i><a href="/wiki/Wikipedia:Link_rot" title="Wikipedia:Link rot"><span title=" Dead link since May 2021">permanent dead link</span></a></i>]</span></sup></span> </li> <li id="cite_note-5"><span class="mw-cite-backlink"><a href="#cite_ref-5">↑</a></span> <span class="reference-text"><link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r9724332" /><cite class="citation web cs1"><a rel="nofollow" class="external text" href="http://www.nasi.com/docs/pdfs/IBM-xSeries-Architecture.pdf">"IBM X-Architecture Technology 2001:A design blueprint for Intel processor-based servers"</a> <span class="cs1-format">(PDF)</span>.</cite><span title="ctx_ver=Z39.88-2004&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Abook&rft.genre=unknown&rft.btitle=IBM+X-Architecture+Technology+2001%3AA+design+blueprint+for+Intel+processor-based+servers&rft_id=http%3A%2F%2Fwww.nasi.com%2Fdocs%2Fpdfs%2FIBM-xSeries-Architecture.pdf&rfr_id=info%3Asid%2Fsimple.wikipedia.org%3ARAID" class="Z3988"></span><sup class="noprint Inline-Template"><span style="white-space: nowrap;">[<i><a href="/wiki/Wikipedia:Link_rot" title="Wikipedia:Link rot"><span title=" Dead link since January 2021">permanent dead link</span></a></i>]</span></sup></span> </li> <li id="cite_note-6"><span class="mw-cite-backlink"><a href="#cite_ref-6">↑</a></span> <span class="reference-text"><link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r9724332" /><cite class="citation web cs1"><a rel="nofollow" class="external text" href="http://www.pcguide.com/ref/hdd/perf/raid/levels/singleLevel7-c.html">"RAID Level 7"</a>.</cite><span title="ctx_ver=Z39.88-2004&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Abook&rft.genre=unknown&rft.btitle=RAID+Level+7&rft_id=http%3A%2F%2Fwww.pcguide.com%2Fref%2Fhdd%2Fperf%2Fraid%2Flevels%2FsingleLevel7-c.html&rfr_id=info%3Asid%2Fsimple.wikipedia.org%3ARAID" class="Z3988"></span></span> </li> <li id="cite_note-7"><span class="mw-cite-backlink"><a href="#cite_ref-7">↑</a></span> <span class="reference-text"><link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r9724332" /><cite class="citation web cs1"><a rel="nofollow" class="external text" href="http://cgi.cse.unsw.edu.au/~neilb/01093607424">"Linux RAID 10 driver"</a>.</cite><span title="ctx_ver=Z39.88-2004&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Abook&rft.genre=unknown&rft.btitle=Linux+RAID+10+driver&rft_id=http%3A%2F%2Fcgi.cse.unsw.edu.au%2F~neilb%2F01093607424&rfr_id=info%3Asid%2Fsimple.wikipedia.org%3ARAID" class="Z3988"></span></span> </li> <li id="cite_note-8"><span class="mw-cite-backlink"><a href="#cite_ref-8">↑</a></span> <span class="reference-text"><link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r9724332" /><cite class="citation web cs1"><a rel="nofollow" class="external text" href="https://web.archive.org/web/20080819054530/http://linux-raid.osdl.org/index.php/Linux_Raid">"Main Page - Linux-raid"</a>. Archived from <a rel="nofollow" class="external text" href="http://linux-raid.osdl.org/index.php/Linux_Raid">the original</a> on 2008-08-19<span class="reference-accessdate">. Retrieved <span class="nowrap">2008-08-18</span></span>.</cite><span title="ctx_ver=Z39.88-2004&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Abook&rft.genre=unknown&rft.btitle=Main+Page+-+Linux-raid&rft_id=http%3A%2F%2Flinux-raid.osdl.org%2Findex.php%2FLinux_Raid&rfr_id=info%3Asid%2Fsimple.wikipedia.org%3ARAID" class="Z3988"></span></span> </li> <li id="cite_note-9"><span class="mw-cite-backlink"><a href="#cite_ref-9">↑</a></span> <span class="reference-text"><link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r9724332" /><cite class="citation web cs1"><a rel="nofollow" class="external text" href="http://blogs.sun.com/bonwick/entry/raid_z">"RAID-Z : Jeff Bonwick's Blog"</a>.</cite><span title="ctx_ver=Z39.88-2004&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Abook&rft.genre=unknown&rft.btitle=RAID-Z+%3A+Jeff+Bonwick%27s+Blog&rft_id=http%3A%2F%2Fblogs.sun.com%2Fbonwick%2Fentry%2Fraid_z&rfr_id=info%3Asid%2Fsimple.wikipedia.org%3ARAID" class="Z3988"></span></span> </li> <li id="cite_note-10"><span class="mw-cite-backlink"><a href="#cite_ref-10">↑</a></span> <span class="reference-text"><link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r9724332" /><cite class="citation web cs1"><a rel="nofollow" class="external text" href="http://blogs.sun.com/ahl/entry/double_parity_raid_z">"Adam Leventhal's Weblog"</a>.</cite><span title="ctx_ver=Z39.88-2004&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Abook&rft.genre=unknown&rft.btitle=Adam+Leventhal%27s+Weblog&rft_id=http%3A%2F%2Fblogs.sun.com%2Fahl%2Fentry%2Fdouble_parity_raid_z&rfr_id=info%3Asid%2Fsimple.wikipedia.org%3ARAID" class="Z3988"></span></span> </li> <li id="cite_note-11"><span class="mw-cite-backlink"><a href="#cite_ref-11">↑</a></span> <span class="reference-text">Jim Gray and Catharine van Ingen, <a rel="nofollow" class="external text" href="http://research.microsoft.com/research/pubs/view.aspx?msr_tr_id=MSR-TR-2005-166">"Empirical Measurements of Disk Failure Rates and Error Rates"</a>, MSTR-2005-166, December 2005</span> </li> <li id="cite_note-schroeder-12"><span class="mw-cite-backlink"><a href="#cite_ref-schroeder_12-0">↑</a></span> <span class="reference-text"><link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r9724332" /><cite id="CITEREFBianca_Schroeder_and_Garth_A._Gibson2007" class="citation web cs1">Bianca Schroeder and Garth A. Gibson (2007). <a rel="nofollow" class="external text" href="http://www.usenix.org/events/fast07/tech/schroeder.html">"Disk Failures in the Real World: What Does an MTTF of 1,000,000 Hours Mean to You?"</a>.</cite><span title="ctx_ver=Z39.88-2004&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Abook&rft.genre=unknown&rft.btitle=Disk+Failures+in+the+Real+World%3A+What+Does+an+MTTF+of+1%2C000%2C000+Hours+Mean+to+You%3F&rft.date=2007&rft.au=Bianca+Schroeder+and+Garth+A.+Gibson&rft_id=http%3A%2F%2Fwww.usenix.org%2Fevents%2Ffast07%2Ftech%2Fschroeder.html&rfr_id=info%3Asid%2Fsimple.wikipedia.org%3ARAID" class="Z3988"></span></span> </li> <li id="cite_note-13"><span class="mw-cite-backlink"><a href="#cite_ref-13">↑</a></span> <span class="reference-text"><link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r9724332" /><cite id="CITEREFJim_Gray" class="citation web cs1">Jim Gray. <a rel="nofollow" class="external text" href="http://research.microsoft.com/~gray/papers/theTransactionConcept.pdf">"The Transaction Concept: Virtues and Limitations (Invited Paper)¦format=pdf"</a> <span class="cs1-format">(PDF)</span>.</cite><span title="ctx_ver=Z39.88-2004&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Abook&rft.genre=unknown&rft.btitle=The+Transaction+Concept%3A+Virtues+and+Limitations+%28Invited+Paper%29%C2%A6format%3Dpdf&rft.au=Jim+Gray&rft_id=http%3A%2F%2Fresearch.microsoft.com%2F~gray%2Fpapers%2FtheTransactionConcept.pdf&rfr_id=info%3Asid%2Fsimple.wikipedia.org%3ARAID" class="Z3988"></span> <link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r9724332" /><cite class="citation web cs1"><a rel="nofollow" class="external text" href="https://web.archive.org/web/20080726180121/http://www.informatik.uni-trier.de/~ley/db/conf/vldb/vldb81.html#Gray81">"VLDB 1981"</a>. Archived from <a rel="nofollow" class="external text" href="http://www.informatik.uni-trier.de/~ley/db/conf/vldb/vldb81.html#Gray81">the original</a> on 2008-07-26<span class="reference-accessdate">. Retrieved <span class="nowrap">2008-08-14</span></span>.</cite><span title="ctx_ver=Z39.88-2004&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Abook&rft.genre=unknown&rft.btitle=VLDB+1981&rft_id=http%3A%2F%2Fwww.informatik.uni-trier.de%2F~ley%2Fdb%2Fconf%2Fvldb%2Fvldb81.html%23Gray81&rfr_id=info%3Asid%2Fsimple.wikipedia.org%3ARAID" class="Z3988"></span> 144-154</span> </li> <li id="cite_note-hdfailure-14"><span class="mw-cite-backlink"><a href="#cite_ref-hdfailure_14-0">↑</a></span> <span class="reference-text"><link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r9724332" /><cite id="CITEREFJim_GrayCatherine_van_Ingen2007" class="citation arxiv cs1">Jim Gray; Catherine van Ingen (2007). "Empirical Measurements of Disk Failure Rates and Error Rates". <a href="/wiki/ArXiv_(identifier)" class="mw-redirect" title="ArXiv (identifier)">arXiv</a>:<span class="id-lock-free" title="Freely accessible"><a rel="nofollow" class="external text" href="https://arxiv.org/abs/cs/0701166">cs/0701166</a></span>.</cite><span title="ctx_ver=Z39.88-2004&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Ajournal&rft.genre=preprint&rft.jtitle=arXiv&rft.atitle=Empirical+Measurements+of+Disk+Failure+Rates+and+Error+Rates&rft.date=2007&rft_id=info%3Aarxiv%2Fcs%2F0701166&rft.au=Jim+Gray&rft.au=Catherine+van+Ingen&rfr_id=info%3Asid%2Fsimple.wikipedia.org%3ARAID" class="Z3988"></span></span> </li> </ol></div> <div class="mw-heading mw-heading2"><h2 id="Other_websites">Other websites</h2><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=RAID&veaction=edit&section=61" title="Change section: Other websites" class="mw-editsection-visualeditor"><span>change</span></a><span class="mw-editsection-divider"> | </span><a href="/w/index.php?title=RAID&action=edit&section=61" title="Edit section's source code: Other websites"><span>change source</span></a><span class="mw-editsection-bracket">]</span></span></div> <style data-mw-deduplicate="TemplateStyles:r9671870">.mw-parser-output .side-box{margin:4px 0;box-sizing:border-box;border:1px solid #aaa;font-size:88%;line-height:1.25em;background-color:var(--background-color-interactive-subtle,#f8f9fa);display:flow-root}.mw-parser-output .side-box-abovebelow,.mw-parser-output .side-box-text{padding:0.25em 0.9em}.mw-parser-output .side-box-image{padding:2px 0 2px 0.9em;text-align:center}.mw-parser-output .side-box-imageright{padding:2px 0.9em 2px 0;text-align:center}@media(min-width:500px){.mw-parser-output .side-box-flex{display:flex;align-items:center}.mw-parser-output .side-box-text{flex:1;min-width:0}}@media(min-width:720px){.mw-parser-output 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