|Initial Release, reviewed by LDP|
|Text corrections, license added.|
This HOWTO describes the usage of SquashFS - a highly-compressed file system for Linux, which is intended for use in tiny/embedded Linux systems, as well as for read-only archiving and general in-file system compression. All steps needed to achieve the result of a mounted and up-and-running squashed file system are given.
Home of this HOWTO
The SquashFS HOWTO lives at http://linuxdoc.artemio.net/squashfs
Any feedback, suggestions and/or contributions are welcome - feel free to send them to: <artemio (at) artemio.net>.
When creating tiny-sized and embedded Linux systems, every byte of the storage device (floppy, flash disk, etc.) is very important, so compression is used everywhere possible. Also, compressed file systems are frequently needed for archiving purposes. For huge public archives, as well as for personal media archives, this is essential.
SquashFS brings all this to a new level. It is a read-only file system that lets you compress whole file systems or single directories, write them to other devices/partitions or to ordinary files, and then mount them directly (if a device) or using a loopback device (if it is a file). The modular, compact system design of SquashFS is bliss. For archiving purposes, SquashFS gives you a lot more flexibility and performance speed than a .tar.gz archive.
SquashFS is distributed as a Linux kernel source patch (which enables SquashFS read support in your kernel), and the mksquashfs tool, which creates squashed file systems (in a file or on a block device).
Data, inodes and directories are compressed
SquashFS stores full uid/gids (32 bits), and file creation time
Files up to 2^32 bytes are supported; file systems can be up to 2^32 bytes
Inode and directory data are highly compacted, and packed on byte boundaries; each compressed inode is on average 8 bytes in length (the exact length varies on file type, i.e. regular file, directory, symbolic link, and block/character device inodes have different sizes)
SquashFS can use block sizes up to 32K, which achieves greater compression ratios than the normal 4K block size
File duplicates are detected and removed
Both big and little endian architectures are supported; SquashFS can mount file systems created on different byte-order machines
To make further chapters clear, the basic steps of getting SquashFS working that we will discuss are:
Patching and recompiling the Linux kernel to enable SquashFS support
Compiling the mksquashfs tool
Creating a compressed file system with mksquashfs
Testing: mounting a squashed file system to a temporary location
Modifying the fstab or startup scripts of your Linux system to mount the squashed file system at boot time
In order to read SquashFS, you need it supported in your kernel - just as if it was a reiserfs or ext3 file system. You have to make sure there is an appropriate patch for your kernel version - it should be located in linux-2.x.y subdirectory of the SquashFS source tree. Also, remember that in most cases you will need a clean (original) Linux kernel source from kernel.org. If your kernel source is from a distro vendor, it may be already pre-patched with custom vendor patches, and patching with a SquashFS patch may raise errors, as SquashFS patches are made against original Linux kernels.
With the 2.x.y kernel source and a proper 2.x.y SquashFS patch present, all you have to do is (we'll assume that you have your Linux kernel source in /usr/src/linux and that you have the SquashFS source in /usr/src/squashfs):
Change to the SquashFS source directory and copy the kernel patch (we'll assume it's named squashfs1.3r2-patch as of the 1.3r2 release) to /usr/src/linux.
bash# cd /usr/src/squashfs bash# cp linux-2.x.y/squashfs1.3r2-patch /usr/src/linux
Go into the directory /usr/src/linux:
bash# cd /usr/src/linux
Patch the Linux kernel:
bash# patch -p1 < squashfs1.3r2-patch
Configure your kernel:
bash# make menuconfig
In the "File systems" section, make sure you enable "Squashed filesystem." If you want to be able to mount the squashed file system via a loopback device in future, you should enable "Loopback device support" in the "Block devices" section.
Now you may compile the kernel and modules:
bash# make dep && make bzImage && make modules && make modules_install
Install your new kernel (name it bzImage-sqsh for convenience, if you like):
bash# cp arch/i386/boot/bzImage /boot/bzImage-sqsh
Modify your boot loader's configuration file to include your new kernel and install the boot loader. Now you may reboot with your new kernel. When it boots, check that everything went fine:
bash# cat /proc/filesystems
You should see the squashfs line among other file systems. This means you have successfully enabled SquashFS in your kernel.
Now you need to compile mksquashfs - the tool for creating squashed file systems.
bash# cd /usr/src/squashfs/squashfs-tools
Compile and install mksquashfs:
bash# make bash# cp mksquashfs /usr/sbin
If everything went fine, typing mksquashfs at the Bash prompt should print it's "usage" message.
The mksquashfs tool can be used for creating new squashed file systems, as well as for appending new data to existing squashed file systems.
The general format for mksquashfs from the command-line is:
bash# mksquashfs source1 source2 ... destination [options] [-e files to exclude]
source items can be files and directories with mixed relative/absolute paths; destination can be regular files (thus, you will have a file system image on your disk), or a block device, such as /dev/fd0 or /dev/hda3.
When the new files are added to the new file system or appended to an existing one, mksquashfs will automatically rename files with duplicate names: if two or more files named text will appear in the same resulting directory, the second file will be renamed to text_1, third one to text_2 and so on. Additionally, duplicate files will be removed, so there will be only one physical instance.
If destination has a pre-existing SquashFS file system on it, by default, the new source items will be appended to existing root directory. Examine the options table below to force mksquashfs to overwrite the whole destination and/or change the way new source items are added.
If a single source file or directory is given, it becomes the root in a newly created file system. If two or more source files and/or directories are given, they will all become sub-items in the root of the new file system.
Files to exclude can be used to specify which files and/or directories you want to omit from the new file system that is to be created. The usage of this option will be described later.
See the next section for more details about mksquashfs options.
All possible options for mksquashfs are shown in the table below.
Table 1. mksquashfs command-line options
|-info||print files, their original size and compression ratio, as they are added to the file system|
|-version||print the version, copyright and licence message|
|-noI or -noInodeCompression||do not compress the inode table|
|-noD or -noDataCompression||do not compress the data|
|-b [block size]||use [block size] filesystem block size (32 Kbytes default) - this can be either 512, 1024, 2048, 4096, 8192, 16384 or 32768|
|-nopad||do not pad the resulting file system to a multiple of 4 KBytes|
|-be or -le||force a big or little endian file system, respectively|
|-check-data||enable additional file system checks|
|-keep-as-directory||if the source is a single directory, force this directory to be a subdirectory of the root in the created file system|
|-noappend||if the destination file/device already contains a squashed file system, overwrite it, rather than append the new data to an existing file system.|
|-root-becomes [name]||can be used while appending to a pre-existing squashed file system, it will make a new root, and [name] directory will contain all pre-existing files/directories.|
In most cases, you should leave all compression/block options by default, as they allow mksquashfs to achieve the best possible compression ratios.
In order to squash a directory (say, /some/dir), and output to a regular file (thus, producing a file system image), you need to perform the following operations:
bash# mksquashfs /some/dir dir.sqsh
mksquashfs will perform the squashing and print the resulting number of inodes and size of data written, as well as the average compression ratio. Now you have your /some/dir directory image in the dir.sqsh file. You can now use the mount command to mount it using a loopback device:
bash# mkdir /mnt/dir bash# mount dir.sqsh /mnt/dir -t squashfs -o loop
To check if you have what's expected:
bash# ls /mnt/dir
If you want to output the file system directly into a device (say, your floppy at /dev/fd0):
#bash mksquashfs /some/dir /dev/fd0
Then just mount the device:
bash# mount /dev/fd0 /mnt/floppy -t squashfs
And check if it's okay:
bash# ls /mnt/floppy
Operations described here correspond to most cases where a read-only compressed file system can be used, whether you want it to be on a block device or in a file. This could be anything from large FTP/HTTP-served archives that don't change often, to having a squashed /usr partition and anything alike with these.
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
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 and use dd to copy /var/arch.sqsh to /dev/hda6:
bash# umount /dev/hda7 bash# dd if=/var/arch.sqsh of=/dev/hda7
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
By saying "tiny/embedded", I mean Linux systems that are being built for booting from floppy disks, IDE/USB flash disks, iso9660 CD-ROMs, small-sized hard drives and the like. Whether you want to have your whole root file system on a single media (a single partition, a single floppy), or have a modular system (several floppies or disk partitions), the procedure is almost identical. Creating such Linux systems themselves is out of scope of this HOWTO - there are dedicated HOWTOs and guides for this (like the Bootdisk HOWTO and Linux From Scratch - visit www.tldp.org to retrieve these documents).
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:
When developing a kernel for your system, make sure you enable SquashFS support so it can mount squashed file systems
Use mksquashfs for creating read-only initial ram disks and/or root and/or other file systems
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
With SquashFS, it is easily possible to squash a full Linux distro complete with XFree86, your favorite desktop manager and other applications onto a single CD-ROM disk. Even a 3.5" CD distro can have all this included.
When following your usual steps for creating a live Linux CD, don't forget to:
Enable SquashFS in your kernel
Create a squashed root file system (read the note below)
Note: if you create a root file system out of a running Linux system, use the -e option for mksquashfs to exclude the /proc file system and the file system image itself that is being created with mksquashfs (I think you know the reasons for these exclusions). What you would use is:
bash# mksquashfs / /tmp/root.sqsh -e /proc /tmp/root.sqsh
I would like to express my sincere thanks and immeasurable respect to:
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