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<H2><A NAME="loopback"></A> <A NAME="s14">14. Using a loopback device</A></H2>
<P>
<P>A loopback device allows a normal disk file to be mounted as a filesystem.
Depending on how much physical memory you have, using a loopback device may
be preferable to using /dev/ram0 for building a root filesystem with Yard.
<P>To use a loopback device you'll need to do the following:
<BLOCKQUOTE>Thanks to Roderich Schupp for some of this information.</BLOCKQUOTE>
<P>
<OL>
<LI>Enable loopback device support in your kernel if you haven't
already. Under <EM>Floppy, IDE and other block devices</EM>, select
either Y or M for <EM>Loopback device support</EM>. Recompile your
kernel and reboot.
</LI>
<LI>Check your
<BLOCKQUOTE>
mount
</BLOCKQUOTE>
manpage to see if it supports
loopback devices. If it doesn't, you'll need modified versions of
mount and losetup. These are available from:
<BLOCKQUOTE>
<A HREF="ftp://ftp.win.tue.nl:/pub/linux/util/">ftp://ftp.win.tue.nl:/pub/linux/util/</A></BLOCKQUOTE>
in the file <CODE>mount-2.5X.tar.gz</CODE>, where X is the latest
version letter.
</LI>
<LI> You'll also probably need mke2fs 1.02 or later. Check the manpage for
<CODE>mke2fs</CODE>; if there is no ``-F'' option listed, you'll need a
newer version. mke2fs is included in the e2fsprogs package
available in:
<BLOCKQUOTE><CODE>
<PRE>
sunsite.unc.edu:/pub/Linux/system/Filesystems/ext2/
</PRE>
</CODE></BLOCKQUOTE>
</LI>
</OL>
<P>To use the loopback device/file with Yard, simply:
<P>
<OL>
<LI>In Config.pl, set $device to the name of a temporary disk file
to hold the disk image, eg /tmp/fsfile. The file need not
already exist. Set $fs_size to its desired size.
</LI>
<LI>Create the $device file:
<BLOCKQUOTE><CODE>
<PRE>
create_loopback_file
</PRE>
</CODE></BLOCKQUOTE>
or do it yourself with:
<BLOCKQUOTE><CODE>
<PRE>
dd if=/dev/zero of=$device bs=1k count=$fs_size
</PRE>
</CODE></BLOCKQUOTE>
and substitute your values for $device and
$fs_size.
</LI>
<LI>If necessary, load the loopback device module (via <CODE>insmod
loop</CODE>) into your kernel.
</LI>
</OL>
<P>Yard may then be run with no further alterations.
<P><EM>Warning:</EM> When using a loopback device, occasionally the ext2
filesystem seems to get corrupted for no apparent reason. If you start
getting errors in the middle of <CODE>make_root_fs</CODE> from sys()
complaining that a directory doesn't exist, this may be what is happening.
I don't know why this happens and I can't reproduce it consistently;
however, unmounting, deleting and re-creating the file seems to get rid of
the problem.
<P>
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