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<HTML ><HEAD ><TITLE >Creating a swap space</TITLE ><META NAME="GENERATOR" CONTENT="Modular DocBook HTML Stylesheet Version 1.63 "><LINK REL="HOME" TITLE="The Linux System Administrator's Guide" HREF="index.html"><LINK REL="UP" TITLE="Memory Management" HREF="memory-management.html"><LINK REL="PREVIOUS" TITLE="What is virtual memory?" HREF="x1752.html"><LINK REL="NEXT" TITLE="Using a swap space" HREF="x1794.html"></HEAD ><BODY CLASS="SECT1" BGCOLOR="#FFFFFF" TEXT="#000000" LINK="#0000FF" VLINK="#840084" ALINK="#0000FF" ><DIV CLASS="NAVHEADER" ><TABLE WIDTH="100%" BORDER="0" CELLPADDING="0" CELLSPACING="0" ><TR ><TH COLSPAN="3" ALIGN="center" >The Linux System Administrator's Guide: Version 0.7</TH ></TR ><TR ><TD WIDTH="10%" ALIGN="left" VALIGN="bottom" ><A HREF="x1752.html" >Prev</A ></TD ><TD WIDTH="80%" ALIGN="center" VALIGN="bottom" >Chapter 7. Memory Management</TD ><TD WIDTH="10%" ALIGN="right" VALIGN="bottom" ><A HREF="x1794.html" >Next</A ></TD ></TR ></TABLE ><HR ALIGN="LEFT" WIDTH="100%"></DIV ><DIV CLASS="SECT1" ><H1 CLASS="SECT1" ><A NAME="AEN1762" >7.2. Creating a swap space</A ></H1 ><P >A swap file is an ordinary file; it is in no way special to the kernel. The only thing that matters to the kernel is that it has no holes, and that it is prepared for use with <B CLASS="COMMAND" >mkswap</B >. It must reside on a local disk, however; it can't reside in a filesystem that has been mounted over NFS due to implementation reasons.</P ><P >The bit about holes is important. The swap file reserves the disk space so that the kernel can quickly swap out a page without having to go through all the things that are necessary when allocating a disk sector to a file. The kernel merely uses any sectors that have already been allocated to the file. Because a hole in a file means that there are no disk sectors allocated (for that place in the file), it is not good for the kernel to try to use them.</P ><P >One good way to create the swap file without holes is through the following command: <TABLE BORDER="1" BGCOLOR="#E0E0E0" WIDTH="100%" ><TR ><TD ><FONT COLOR="#000000" ><PRE CLASS="SCREEN" > <TT CLASS="PROMPT" >$</TT > <TT CLASS="USERINPUT" ><B >dd if=/dev/zero of=/extra-swap bs=1024 count=1024</B ></TT > <TT CLASS="COMPUTEROUTPUT" >1024+0 records in 1024+0 records out</TT > <TT CLASS="PROMPT" >$</TT > </PRE ></FONT ></TD ></TR ></TABLE > where <TT CLASS="FILENAME" >/extra-swap</TT > is the name of the swap file and the size of is given after the <TT CLASS="LITERAL" >count=</TT >. It is best for the size to be a multiple of 4, because the kernel writes out <I CLASS="GLOSSTERM" >memory pages</I >, which are 4 kilobytes in size. If the size is not a multiple of 4, the last couple of kilobytes may be unused.</P ><P >A swap partition is also not special in any way. You create it just like any other partition; the only difference is that it is used as a raw partition, that is, it will not contain any filesystem at all. It is a good idea to mark swap partitions as type 82 (Linux swap); this will the make partition listings clearer, even though it is not strictly necessary to the kernel.</P ><P >After you have created a swap file or a swap partition, you need to write a signature to its beginning; this contains some administrative information and is used by the kernel. The command to do this is <B CLASS="COMMAND" >mkswap</B >, used like this: <TABLE BORDER="1" BGCOLOR="#E0E0E0" WIDTH="100%" ><TR ><TD ><FONT COLOR="#000000" ><PRE CLASS="SCREEN" > <TT CLASS="PROMPT" >$</TT > <TT CLASS="USERINPUT" ><B >mkswap /extra-swap 1024</B ></TT > <TT CLASS="COMPUTEROUTPUT" >Setting up swapspace, size = 1044480 bytes</TT > <TT CLASS="PROMPT" >$</TT > </PRE ></FONT ></TD ></TR ></TABLE > Note that the swap space is still not in use yet: it exists, but the kernel does not use it to provide virtual memory.</P ><P >You should be very careful when using <B CLASS="COMMAND" >mkswap</B >, since it does not check that the file or partition isn't used for anything else. <EM >You can easily overwrite important files and partitions with <B CLASS="COMMAND" >mkswap</B >!</EM > Fortunately, you should only need to use <B CLASS="COMMAND" >mkswap</B > when you install your system.</P ><P >The Linux memory manager limits the size of each swap space to about 127 MB (for various technical reasons, the actual limit is (4096-10) * 8 * 4096 = 133890048$ bytes, or 127.6875 megabytes). You can, however, use up to 8 swap spaces simultaneously, for a total of almost 1 GB. <A NAME="AEN1790" HREF="#FTN.AEN1790" >[1]</A > </P ><P >This is actually no longer true, this section is slated for a rewrite Real Soon Now (tm). With newer kernels and versions of the <B CLASS="COMMAND" >mkswap</B > command the actual limit depends on architecture. For i386 and compatibles it is 2Gigabytes, other architectures vary. Consult the mkswap(8) manual page for more details.</P ></DIV ><H3 CLASS="FOOTNOTES" >Notes</H3 ><TABLE BORDER="0" CLASS="FOOTNOTES" WIDTH="100%" ><TR ><TD ALIGN="LEFT" VALIGN="TOP" WIDTH="5%" ><A NAME="FTN.AEN1790" HREF="x1762.html#AEN1790" >[1]</A ></TD ><TD ALIGN="LEFT" VALIGN="TOP" WIDTH="95%" ><P >A gigabyte here, a gigabyte there, pretty soon we start talking about real memory.</P ></TD ></TR ></TABLE ><DIV CLASS="NAVFOOTER" ><HR ALIGN="LEFT" WIDTH="100%"><TABLE WIDTH="100%" BORDER="0" CELLPADDING="0" CELLSPACING="0" ><TR ><TD WIDTH="33%" ALIGN="left" VALIGN="top" ><A HREF="x1752.html" >Prev</A ></TD ><TD WIDTH="34%" ALIGN="center" VALIGN="top" ><A HREF="index.html" >Home</A ></TD ><TD WIDTH="33%" ALIGN="right" VALIGN="top" ><A HREF="x1794.html" >Next</A ></TD ></TR ><TR ><TD WIDTH="33%" ALIGN="left" VALIGN="top" >What is virtual memory?</TD ><TD WIDTH="34%" ALIGN="center" VALIGN="top" ><A HREF="memory-management.html" >Up</A ></TD ><TD WIDTH="33%" ALIGN="right" VALIGN="top" >Using a swap space</TD ></TR ></TABLE ></DIV ></BODY ></HTML >