SquashFS HOWTO

Artemiy I. Pavlov

[http://artemiolabs.com/] ArtemioLabs

<ap (at) artemio (dot) net>

2005-03-25
Revision History                                                             
Revision 1.7                            2005-03-25                           
Changes according to SquashFS release 2.1.                                   
Revision 1.6                            2004-11-10                           
Changes according to SquashFS release 2.0. Text corrections.                 
Revision 1.5                            2004-06-07                           
Changes according to SquashFS release 2.0 alpha. Lots of description         
improvements and clarifications. Split instructions for Linux kernels of     
2.6.x (new) and 2.4.x series.                                                
Revision 1.1                            2004-05-22                           
Changes according to SquashFS release 1.3r3.                                 
Revision 1.0                            2004-02-19                           
Initial Release, reviewed by LDP.                                            
Revision 0.2                            2003-12-08                           
Text corrections, license added.                                             
Revision 0.1                            2003-11-24                           
Initial version. Instructions for SquashFS release 1.3r2.                    


Abstract

This HOWTO describes the usage of SquashFS - a highly-compressed read-only
file system for Linux, which is intended for use in tiny-sized and embedded
systems, and anywhere else you'd want to use a compressed file system. With
this document, you'll learn how to prepare a SquashFS-ready Linux kernel,
create a sqaushed file system and happily use it.


Home of this HOWTO

  The SquashFS HOWTO lives at [http://artemio.net/projects/linuxdoc/squashfs]
http://artemio.net/projects/linuxdoc/squashfs. There you will always find the
latest version of the document, and will be able to send your feedback.
-----------------------------------------------------------------------------

Table of Contents
1. What is SquashFS
    1.1. Introduction
    1.2. Overview of SquashFS
    1.3. Making it clear
   
   
2. Getting ready for SquashFS
    2.1. Acquiring SquashFS
    2.2. Preparing a SquashFS-capable kernel
    2.3. Compiling the mksquashfs tool
   
   
3. The mksquashfs tool, exposed
    3.1. Using mksquashfs
    3.2. Command-line options
   
   
4. Creating and using squashed file systems
    4.1. Basic steps
    4.2. Squashing file systems
    4.3. Creating tiny/embedded systems
   
   
5. Acknowledgements
6. License

-----------------------------------------------------------------------------
1. What is SquashFS

1.1. Introduction

 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).

 The latest SquashFS release tree is 2.x, the former one was 1.x. This
document describes both these releases with proper notes given. For example,
if some feature or parameter is different in these release trees, it will be
written as follows: new value (2.x) or old value (1.x)
-----------------------------------------------------------------------------

1.2. Overview of SquashFS

  * 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 32 Kb (1.x) and 64Kb (2.x), which
    achieves greater compression ratios than the normal 4K block size
  * SquashFS 2.x inroduced the concept of fragment blocks: an ability to join
    multiple files smaller than block size into a single block, achieving
    greater compression ratios
  * 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

-----------------------------------------------------------------------------
1.3. Making it clear

 Now let's make sure any further discussions will be clearer fro you to
understand. The procedure of getting SquashFS working, basically, consists of
the following steps:

 1. Patching and recompiling the target Linux kernel to enable SquashFS
    support
 2. Compiling the mksquashfstool
 3. Creating a compressed file system with mksquashfs
 4. Testing: mounting a squashed file system to a temporary location
 5. Modifying the /etc/fstabor startup scripts of your target Linux system to
    mount the new squashed file system when needed

-----------------------------------------------------------------------------
2. Getting ready for SquashFS

2.1. Acquiring SquashFS

 The SquashFS home site is located at [http://squashfs.sourceforge.net/]
http://squashfs.sourceforge.net/ - it contains news for the latest release
and it's changelog, as well as general information about SquashFS. You can
grab the latest version at the SqaushFS [http://sourceforge.net/project/
showfiles.php?group_id=63835] project page at SourceForge.
-----------------------------------------------------------------------------

2.2. Preparing a SquashFS-capable kernel

 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 [http://
kernel.org/] 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 will almost surely not work, as SquashFS patches are made
against original Linux kernels.
-----------------------------------------------------------------------------

2.2.1. Patching the kernel source

 With a kernel source and a proper 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 squashfs-patch) to /usr/src/linux.
        bash# cd /usr/src/squashfs                                           
        bash# cp linux-2.x.y/squashfs-patch /usr/src/linux                   

 Go to the linux kernel source directory /usr/src/linux:
        bash# cd /usr/src/linux                                              

Note: please remember that we will not be leaving this directory during all
further kernel-related procedures, and all paths will be given relative to /
usr/src/linux.

Now patch the source with the SquashFS patch:
        bash# patch -p1 < squashfs-patch                                     
-----------------------------------------------------------------------------

2.2.2. Compiling a 2.6.x kernel

Cleanup and prepare the kernel source:
        bash# make distclean                                                 
        bash# make mrproper                                                  

Configure the kernel using your favourite method (config/menuconfig/xconfig/
gconfig):
        bash# make menuconfig                                                

 1. In the "File systems"section, "Miscellaneous file systems"subsection,
    enable the "Squashed filesystem"option, whether as module or bundled with
    the kernel. It is only obligatory to compile SquashFS inside the kernel
    if you plan using squashed initial RAM disks (initrd).
 2. If you would like to use a squashed initial RAM disk, enable the "Initial
    RAM disk support"in the "Device drivers"section, "Block devices"
    subsection.
 3. If you want to be able to mount the squashed file system via a loopback
    devicein future, you should enable "Loopback device support"in the "
    Device drivers"section, "Block devices"subsection.

Now you may compile the kernel and modules:
        bash# make                                                           
-----------------------------------------------------------------------------

2.2.3. Compiling a 2.4.x kernel

Configure the kernel:
        bash# make menuconfig                                                

 1. In the "File systems"section, enable the "Squashed filesystem"option,
    whether as module or bundled with the kernel. It is only obligatory to
    compile SquashFS inside the kernel if you plan using squashed initial RAM
    disks (initrd).
 2. If you would like to use a squashed initial RAM disk, enable the "Initial
    RAM disk support"in the "Block devices"section.
 3. If you want to be able to mount the squashed file system via a loopback
    devicein future, you should enable "Loopback device support"in the "Block
    devices"section.

Now you may compile the kernel and modules:
        bash# make dep                                                       
        bash# make bzImage                                                   
        bash# make modules                                                   
-----------------------------------------------------------------------------

2.2.4. Installing and testing the kernel

It's time to install your new SquashFS-enabled kernel. The instructions below
are for installing and booting the kernel on the host machine. You may want
to install and test it on the target system.

We assume that the kernel was compiled for a x86 architecture, and the
compressed kernel image is located in the arch/i386/boot/ subdirectory of the
kernel tree. Now copy the kernel to the /boot directory (and name it
bzImage-sqsh for convenience, if you like):
        bash# cp arch/i386/boot/bzImage /boot/bzImage-sqsh                   

 Don't forget to install the kernel modules if you have any:
        bash# make modules_install                                           

 Modify your boot loader's configuration file to include your new kernel and
install (update) the boot loader. Now you may reboot with your new kernel.
When it boots, check that everything went fine:
        bash# cat /proc/filesystems                                          

 Or, if you built SquashFS support as a kernel module:
        bash# insmod squashfs                                                
        bash# cat /proc/filesystems                                          

 If you see the squashfs line among other file systems, this means you have
successfully enabled SquashFS in your kernel.
-----------------------------------------------------------------------------

2.3. Compiling the mksquashfs tool

 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 shell prompt should print
it's "usage" message.
-----------------------------------------------------------------------------

3. The mksquashfs tool, exposed

3.1. Using mksquashfs

 mksquashfs is the tool for creating new squashed file systems, and for
appending new data to existing squashed file systems. The general
command-line format for mksquashfs is:
        bash# mksquashfs source1 source2 ... destination [options]           

  * source1, source2, etc.: files and directories to be added to the
    resulting filke system, given with relative and/or absolute paths
  * destination: a regular file (filesystem image file), or a block device
    (such as /dev/fd0or /dev/hda3) where you want to have your squashed file
    system

  Notes for default mksquashfs behavior:

  * When the new files are added to the new file system or appended to an
    existing one, mksquashfswill automatically rename files with duplicate
    names: if two or more files named textwill appear in the same resulting
    directory, the second file will be renamed to text_1, third one to
    text_2and so on.
  * Duplicate files will be removed, so there will be only one physical
    instance (with SquashFS 2.x, you can disable the detection/rtemoval of
    the duplicates with the -no-duplicatesoption).
  * If destinationhas a pre-existing SquashFS file system on it, by default,
    the new sourceitems will be appended to the existing root directory.
    Examine the options table below to force mksquashfsto overwrite the whole
    destination and/or change the way new source items are added. Please note
    that it is not possible to append to a file system created with 
    mksquashfs1.x using mksquashfs2.x. You will need to mount the
    SquashFS-1.x file system and copy the files to some location, and then
    join them with other needed files to create a SquashFS-2.x file system.
  * 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.
  * The resulting filesystem will be padded to a multiple of 4 Kb: this is
    required for filesystems to be used on block devices. If you are very
    sure you don't ned this, use the -nopadoption to disable this operation.

 See the next section for more details about all possible options.
-----------------------------------------------------------------------------

3.2. Command-line options

 All possible options for mksquashfs are shown in the table below.


Table 1. Command-line options of the mksquashfs tool
+----------------------+----------------------------------------------------+
|Option                |Description                                         |
+----------------------+----------------------------------------------------+
|-2.0                  |force mksquashfs version 2.1 to create a version 2.0|
|                      |filesystem                                          |
+----------------------+----------------------------------------------------+
|-all-root or          |make all files in the target file system owned by   |
|-root-owned           |root (UID=0, GID=0)                                 |
+----------------------+----------------------------------------------------+
|-always-use-fragments |divide all files greater than block size into       |
|                      |fragments (2.x only, will result in greater         |
|                      |compression ratios)                                 |
+----------------------+----------------------------------------------------+
|-b [block size]       |use [block size] filesystem block size (32 Kbytes   |
|                      |default) - this can be either 512, 1024, 2048, 4096,|
|                      |8192, 16384 or 32768                                |
+----------------------+----------------------------------------------------+
|-be or -le            |force a big or little endian file system,           |
|                      |respectively                                        |
+----------------------+----------------------------------------------------+
|-check-data           |enable additional file system checks                |
+----------------------+----------------------------------------------------+
|-e [file1] ( [file2]  |specify which files and/or directories to omit from |
|... )                 |the new file system that is to be created           |
+----------------------+----------------------------------------------------+
|-ef [file]            |specify a file which contains the list of files/    |
|                      |directories to exclude                              |
+----------------------+----------------------------------------------------+
|-force-gid [GID]      |set all group IDs in target file system to [GID]    |
|                      |(can be specified as a name or a number)            |
+----------------------+----------------------------------------------------+
|-force-uid [UID]      |set all user IDs in target file system to [UID] (can|
|                      |be specified as a name or a number)                 |
+----------------------+----------------------------------------------------+
|-info                 |print files, their original size and compression    |
|                      |ratio, as they are added to the file system         |
+----------------------+----------------------------------------------------+
|-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      |
+----------------------+----------------------------------------------------+
|-no-duplicates        |do not detect/remove duplicate file names           |
+----------------------+----------------------------------------------------+
|-noD or               |do not compress the data                            |
|-noDataCompression    |                                                    |
+----------------------+----------------------------------------------------+
|-noF or               |do not compress the fragments (2.x only)            |
|-noFragmentCompression|                                                    |
+----------------------+----------------------------------------------------+
|-no-fragments         |do not generate fragment blocks (2.x only, this will|
|                      |produce almost the same filesystem as 1.x did)      |
+----------------------+----------------------------------------------------+
|-noI or               |do not compress the inode table                     |
|-noInodeCompression   |                                                    |
+----------------------+----------------------------------------------------+
|-nopad                |do not pad the resulting file system to a multiple  |
|                      |of 4 KBytes                                         |
+----------------------+----------------------------------------------------+
|-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                                        |
+----------------------+----------------------------------------------------+
|-version              |print the version, copyright and license message    |
+----------------------+----------------------------------------------------+

 In most cases, you should leave all compression/block options by default, as
they allow mksquashfs to achieve the best possible compression ratios.
-----------------------------------------------------------------------------

4. Creating and using squashed file systems

4.1. Basic steps

 In order to create a squashed file system out of a single directory (say, /
some/dir), and output it to a regular file (thus, producing a file system
image), you need to say only one magic phrase:
        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                                                 
-----------------------------------------------------------------------------

4.2. Squashing file systems

 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.
-----------------------------------------------------------------------------

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. Creating tiny/embedded systems

 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 [http://www.tldp.org] www.tldp.org to
retrieve these documents).
-----------------------------------------------------------------------------

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).
-----------------------------------------------------------------------------

5. Acknowledgements

 I would like to express my sincere thanks and immeasurable respect to:

  * Phillip Lougher - for his brilliant work under squashfs, for creating an
    exculsive patch for linux-2.4.18, for his help with polishing this howto
    and answers to my mails
  * Tabatha Marshall at TLDP for helping me with bringing this HOWTO to the
    final 1.0 release
  * Everybody at [http://www.tldp.org] The Linux Documentation Projectfor
    their great work under all the HOWTOs and guides that helped me a lot
    with exploring and hacking Linux
  * All those at the TLDP mailing lists who helped me with getting started
  * Endless thanks and respect to everybody who develops open-source software

-----------------------------------------------------------------------------
6. License

 This document may be used and distributed under the terms and conditions set
forth in the Open Content licence. In short, this means that you can freely
modify and re-distribute the HOWTO under the main condition that you keep the
author and copyright the article along. The full text of the licence is
available at [http://www.opencontent.org/opl.shtml] http://
www.opencontent.org/opl.shtml