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diff --git a/sag-0.6.1-www/sag-0.6.1.html/x1466.html b/sag-0.6.1-www/sag-0.6.1.html/x1466.html new file mode 100644 index 0000000..c41cecd --- /dev/null +++ b/sag-0.6.1-www/sag-0.6.1.html/x1466.html @@ -0,0 +1,290 @@ +<!DOCTYPE HTML PUBLIC "-//Norman Walsh//DTD DocBook HTML 1.0//EN"> +<HTML +><HEAD +><TITLE +>Creating a swap space</TITLE +><META +NAME="GENERATOR" +CONTENT="Modular DocBook HTML Stylesheet"><LINK +REL="HOME" +TITLE="The Linux System Administrators' Guide" +HREF="book1.html"><LINK +REL="UP" +TITLE="Memory Management" +HREF="c1450.html"><LINK +REL="PREVIOUS" +TITLE="Memory Management" +HREF="c1450.html"><LINK +REL="NEXT" +TITLE="Using a swap space" +HREF="x1495.html"></HEAD +><BODY +BGCOLOR="#FFFFFF" +TEXT="#000000" +><DIV +CLASS="NAVHEADER" +><TABLE +WIDTH="100%" +BORDER="0" +CELLPADDING="0" +CELLSPACING="0" +><TR +><TH +COLSPAN="3" +ALIGN="center" +>The Linux System Administrators' Guide</TH +></TR +><TR +><TD +WIDTH="10%" +ALIGN="left" +VALIGN="bottom" +><A +HREF="c1450.html" +>Prev</A +></TD +><TD +WIDTH="80%" +ALIGN="center" +VALIGN="bottom" +>Chapter 5. Memory Management</TD +><TD +WIDTH="10%" +ALIGN="right" +VALIGN="bottom" +><A +HREF="x1495.html" +>Next</A +></TD +></TR +></TABLE +><HR +ALIGN="LEFT" +WIDTH="100%"></DIV +><DIV +CLASS="SECT1" +><H1 +CLASS="SECT1" +><A +NAME="AEN1466" +>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: + +<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 +> + + 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 \cmd{mkswap}, used like this: + +<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 +> + + 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. <I +CLASS="EMPHASIS" +>You + can easily overwrite important files and partitions with + <B +CLASS="COMMAND" +>mkswap</B +>!</I +> 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 + 16 swap spaces simultaneously, for a total of almost + 2 GB. + + <A +NAME="AEN1493" +HREF="#FTN.AEN1493" +>[1]</A +> + + </P +></DIV +><H3 +>Notes</H3 +><TABLE +BORDER="0" +CLASS="FOOTNOTES" +WIDTH="100%" +><TR +><TD +ALIGN="LEFT" +VALIGN="TOP" +WIDTH="5%" +><A +NAME="FTN.AEN1493" +HREF="x1466.html#AEN1493" +>[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="c1450.html" +>Prev</A +></TD +><TD +WIDTH="34%" +ALIGN="center" +VALIGN="top" +><A +HREF="book1.html" +>Home</A +></TD +><TD +WIDTH="33%" +ALIGN="right" +VALIGN="top" +><A +HREF="x1495.html" +>Next</A +></TD +></TR +><TR +><TD +WIDTH="33%" +ALIGN="left" +VALIGN="top" +>Memory Management</TD +><TD +WIDTH="34%" +ALIGN="center" +VALIGN="top" +><A +HREF="c1450.html" +>Up</A +></TD +><TD +WIDTH="33%" +ALIGN="right" +VALIGN="top" +>Using a swap space</TD +></TR +></TABLE +></DIV +></BODY +></HTML +>
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