4. Creating and using squashed file systems

4.2.1. Example 1

Let's suppose you have a /var/arch directory with lots of files and that you want to turn it into a squashed file system and keep it on your root partition as a file (it will be a file system image that you will mount via a loopback device). The operations needed to perform are as follows.

Squash the directory, then mount it via loopback to test it:

	bash# mksquashfs /var/arch /var/arch.sqsh
	bash# mkdir /mnt/tmp
	bash# mount /var/arch.sqsh /mnt/tmp -t squashfs -o loop
	bash# ls /mnt/tmp

If everything is as expected, make this file system mount automatically at boot time by adding this line to your /etc/fstab:

	/var/arch.sqsh	/var/arch	squashfs	ro,defaults	0 0

Unmount the file system from the temporary mount point, and mount using it's fstab entry:

	bash# umount /mnt/tmp
	bash# mount /var/arch

Now just ensure that everything works fine:

	bash# ls /var/arch

4.2.2. Example 2

Say you have two hard disk partitions, /dev/hda6 (which is empty) and /dev/hda7 (which is bigger than /dev/hda6, mounted at /var/arch, contains some data and is full). Now, say you want to squash the /dev/hda7 file system and move it to /dev/hda6, then use /dev/hda7 for some other purposes. We will suppose you have the following line in /etc/fstab (reiserfs is just an example file system used on /dev/hda7):

	/dev/hda7	/var/arch	reiserfs	defaults	0 0

In the same fashion as with the previous example:

	bash# mksquashfs /var/arch /var/arch.sqsh
	bash# mkdir /mnt/tmp
	bash# mount /var/arch.sqsh /mnt/tmp -t squashfs -o loop
	bash# ls /mnt/tmp

If everything went fine, unmount /dev/hda7 (if needed) and use dd to copy /var/arch.sqsh to /dev/hda6:

	bash# umount /dev/hda7
	bash# dd if=/var/arch.sqsh of=/dev/hda6

Now change the line in /etc/fstab for /dev/hda7 to:

	/dev/hda6	/var/arch	squashfs	ro,defaults	0 0

Mount the new file system and check to see if all went fine:

	bash# mount /var/arch
	bash# ls /var/arch

Don't forget to erase the unneeded file system image:

	bash# rm /var/arch.sqsh

4.3.1. Squashed file systems on floppy/flash/hard disks

In order to use SquashFS for creating Linux systems on small disks, you just have to follow the usual steps for creating a minimal system, performing the following operations at respective points:

1. When developing a kernel for your system, make sure you enable SquashFS support so it can mount squashed file systems

2. Use mksquashfs for creating read-only initial ram disks and/or root and/or other file systems

3. Don't forget to set file system types to squashfs in /etc/fstab and/or the startup scripts of your system for mounting squashed file systems

Floppy example. Let's say you have your floppy system tree at /home/user/floppylinux and you want to place the root file system on one floppy and /usr on another. What you should do is:

	bash# cd /home/user
	bash# mksquashfs floppylinux root.sqsh -e usr
	bash# mksquashfs floppylinux/usr usr.sqsh

Note 1: you can see here how we use the -e option to exclude the /usr directory for root file system's image.

Note 2: don't forget to specify squashfs in your root disk's /etc/fstab or startup scripts when mounting the /usr file system.

Insert a root disk in your 3.5" floppy drive (I assume you have a lilo or grub on it, and, thus, a file system exists on this floppy, and the root file system will reside under the /boot directory of this file system):

	bash# mount /mnt/floppy
	bash# cp root.sqsh /mnt/floppy/boot

When done, unmount the root floppy, change the floppy to a /usr disk and use dd to transfer the usr file system:

	bash# dd if=usr.sqsh of=/dev/fd0

4.3.2. Squashed file systems on CD-ROMs

With SquashFS, you can compress large file systems that will be used in live CDs (just as an example).

1. Enable SquashFS in the linux kernel of the target system
2. Create a squashed root file system
3. Modify the /etc/fstabor startup scripts of the target system to mount the squashd file system when you need it

If you create a root file system out of a running Linux system, use the -e option for mksquashfs to exclude all pseudo-filesystems such as /proc, /sys (on linux kernels after 2.5.x) and /dev (when using DevFS). Also, don't forget to add the file system image itself that is being created with mksquashfs (I think you know the reasons for these exclusions).