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><H1
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><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
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><TD
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><A
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HREF="x1466.html#AEN1493"
>[1]</A
></TD
><TD
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VALIGN="TOP"
WIDTH="95%"
><P
>A gigabyte here, a gigabyte there, pretty
soon we start talking about real memory.</P
></TD
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