When installing a modern Linux distribution on older computers, one problem you may face is the lack of disk space. I ran into this last week, while helping a friend install Ubuntu on an antique laptop with a 2G hard drive. The obvious starting point is to begin with a minimalist installation — Ubuntu Alternate CD (my choice), Arch Linux, or a few others. The good news is that your system doesn’t have to stay minimalistic if you know how to tailor the distribution.

One way to save space is to use data compression. It’s possible to keep parts of the filesystem compressed on disk and have Linux decompress them on the fly when they’re needed. This ideea is as old as Stacker / DoubleSpace, but for Linux we need to do more work, as there’s no stable read-write compressed filesystem as of this writing (though you may want to watch Johan Parent’s compFUSEd as it matures).

First, install the tools: squashfs, a compressed file system that yields better performance than the traditional cramfs, and autofs, to mount and unmount compressed directories automatically. Next, if you’ve never used a compressed filesystem, it helps to play with squashfs a bit:

# log in as root or type "sudo bash"
mksquashfs /tmp dummy.squashfs
mount -o loop dummy.squashfs /mnt
ls /mnt           # should be identical to /tmp
touch /mnt/x  # won't work, squashfs is read-only

This example creates a squash file system (in the file dummy.squashfs) that mirrors the contents of /tmp and mounts it (using loop, since it’s an ordinary file and not a block device) on /mnt. As the last command demonstrates, you can’t write in a squashfs, so you’ll want to compress directories that are normally not modified (so /tmp would actually be a bad choice, and so would be any user home directory, /var etc.)

Now, to work — let’s set autofs up (this only needs to be done once:)

cd /etc
echo '/var/autofs/squash /etc/auto.z --timeout=300' >>auto.master
echo '* -fstype=squashfs,loop :/opt/squashfs/&.squashfs' >>auto.z
/etc/init.d/autofs restart

The first line tells autofs to read /etc/auto.z (and to unmount auto-mounted directories 300 seconds after they are unused for 300 seconds); the second one says that whenever someone accesses /var/autofs/squash/DIR (where DIR is an arbitrary name), autofs should try to mount /opt/squashfs/DIR.squashfs automatically.

Next, set your sights on a large, read-only directory — say /usr/lib/mozilla-thunderbird. Here’s the plan:

1. convert relative symlinks: for f in `find /usr/lib/mozilla-thunderbird -type l`; do t=`readlink -f $f`; rm $f; ln -s $t $f; done
2. create a compressed filesystem: mksquashfs /usr/lib/mozilla-thunderbird /opt/squashfs/mozilla-thunderbird-lib.squash
3. remove the original directory: rm -rf /usr/lib/mozilla-thunderbird
4. replace the directory with a symbolic link: ln -s /var/autofs/squash/mozilla-thunderbird-lib /usr/lib/mozilla-thunderbird

You may wonder why the first step is necessary. The answer is that /usr/lib/mozilla-thunderbird contains some relative links (things like ../share/icons) that would break when the directory is relocated to /var/autofs/squash. So we use find to locate symlinks, readlink to read their target, and then rewrite these links.

That’s it. Whenever you access the compressed directory, it will be automounted:

ls /usr/lib/mozilla-thunderbird
mount

This method does have one disadvantage: if you ever upgrade thunderbird, dpkg will follow the compressed directory symlink and try to write inside it (which will fail). You should remove the /usr/lib/mozilla-thunderbird symlink prior to an upgrade (and, presumably, re-compress once the upgrade completes)