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	SQUASHFS 2.2 - A squashed read-only filesystem for Linux

	Copyright 2005 Phillip Lougher (phillip@lougher.demon.co.uk)

	Released under the GPL licence (version 2 or later).

Welcome to Squashfs version 2.2-r2.  Please see the CHANGES file for details
of changes.

Squashfs is a highly compressed read-only filesystem for Linux.
It uses zlib compression to compress both files, inodes and directories.
Inodes in the system are very small and all blocks are packed to minimise
data overhead. Block sizes greater than 4K are supported up to a maximum
of 64K.

Squashfs is intended for general read-only filesystem use, for archival
use (i.e. in cases where a .tar.gz file may be used), and in constrained
block device/memory systems (e.g. embedded systems) where low overhead is


1. Data, inodes and directories are compressed.

2. Squashfs stores full uid/gids (32 bits), and file creation time.

3. Files up to 2^32 bytes are supported.  Filesystems can be up to
   2^32 bytes.

4. 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/char device inodes have different sizes).

5. Squashfs can use block sizes up to 64K (the default size is 64K).
   Using 64K blocks achieves greater compression ratios than the normal
   4K block size.

6. File duplicates are detected and removed.

7. Both big and little endian architectures are supported.  Squashfs can
   mount filesystems created on different byte order machines.


Squashfs filesystems should be mounted with 'mount' with the filesystem type
'squashfs'.  If the filesystem is on a block device, the filesystem can be
mounted directly, e.g.

%mount -t squashfs /dev/sda1 /mnt

Will mount the squashfs filesystem on "/dev/sda1" under the directory "/mnt".

If the squashfs filesystem has been written to a file, the loopback device
can be used to mount it (loopback support must be in the kernel), e.g.

%mount -t squashfs image /mnt -o loop

Will mount the squashfs filesystem in the file "image" under
the directory "/mnt".


3.1 Mksquashfs options and overview.

As squashfs is a read-only filesystem, the mksquashfs program must be used to
create populated squashfs filesystems.

SYNTAX:mksquashfs source1 source2 ...  dest [options] [-e list of exclude

Options are
-version		print version, licence and copyright message
-info			print files written to filesystem
-b <block_size>		set data block to <block_size>.  Default 65536 bytes
-2.0			create a 2.0 filesystem
-noI			do not compress inode table
-noD			do not compress data blocks
-noF			do not compress fragment blocks
-no-fragments		do not use fragments
-always-use-fragments	use fragment blocks for files larger than block size
-no-duplicates		do not perform duplicate checking
-noappend		do not append to existing filesystem
-keep-as-directory	if one source directory is specified, create a root
			directory containing that directory, rather than the
			contents of the directory
-root-becomes <name>	when appending source files/directories, make the
			original root become a subdirectory in the new root
			called <name>, rather than adding the new source items
			to the original root
-all-root		make all files owned by root
-force-uid uid		set all file uids to uid
-force-gid gid		set all file gids to gid
-le			create a little endian filesystem
-be			create a big endian filesystem
-nopad			do not pad filesystem to a multiple of 4K
-check_data		add checkdata for greater filesystem checks
-root-owned		alternative name for -all-root
-noInodeCompression	alternative name for -noI
-noDataCompression	alternative name for -noD
-noFragmentCompression	alternative name for -noF
-sort <sort_file>	sort files according to priorities in <sort_file>.  One
			file or dir with priority per line.  Priority -32768 to
			32767, default priority 0
-ef <exclude_file>	list of exclude dirs/files.  One per line

Source1 source2 ... are the source directories/files containing the
files/directories that will form the squashfs filesystem.  If a single
directory is specified (i.e. mksquashfs source output_fs) the squashfs
filesystem will consist of that directory, with the top-level root
directory corresponding to the source directory.

If multiple source directories or files are specified, mksquashfs will merge
the specified sources into a single filesystem, with the root directory
containing each of the source files/directories.  The name of each directory
entry will be the basename of the source path.   If more than one source
entry maps to the same name, the conflicts are named xxx_1, xxx_2, etc. where
xxx is the original name.

To make this clear, take two example directories.  Source directory
"/home/phillip/test" contains  "file1", "file2" and "dir1".
Source directory "goodies" contains "goodies1", "goodies2" and "goodies3".

usage example 1:

%mksquashfs /home/phillip/test output_fs

This will generate a squashfs filesystem with root entries
"file1", "file2" and "dir1".

example 2:

%mksquashfs /home/phillip/test goodies output_fs

This will create a squashfs filesystem with the root containing
entries "test" and "goodies" corresponding to the source
directories "/home/phillip/test" and "goodies".

example 3:

%mksquashfs /home/phillip/test goodies test output_fs

This is the same as the previous example, except a third
source directory "test" has been specified.  This conflicts
with the first directory named "test" and will be renamed "test_1".

Multiple sources allow filesystems to be generated without needing to
copy all source files into a common directory.  This simplifies creating

The -keep-as-directory option can be used when only one source directory
is specified, and you wish the root to contain that directory, rather than
the contents of the directory.  For example:

example 4:

%mksquashfs /home/phillip/test output_fs -keep-as-directory

This is the same as example 1, except for -keep-as-directory.
This will generate a root directory containing directory "test",
rather than the "test" directory contents "file1", "file2" and "dir1".

The Dest argument is the destination where the squashfs filesystem will be
written.  This can either be a conventional file or a block device.  If the file
doesn't exist it will be created, if it does exist and a squashfs
filesystem exists on it, mksquashfs will append.  The -noappend option will
write a new filesystem irrespective of whether an existing filesystem is

3.2 Changing compression defaults used in mksquashfs

There are a large number of options that can be used to control the 
compression in mksquashfs.  By and large the defaults are the most
optimum settings and should only be changed in exceptional circumstances!

The -noI, -noD and -noF options (also -noInodeCompression, -noDataCompression
and -noFragmentCompression) can be used to force mksquashfs to not compress
inodes/directories, data and fragments respectively.  Giving all options
generates an uncompressed filesystem.

The -no-fragments tells mksquashfs to not generate fragment blocks, and rather
generate a filesystem similar to a Squashfs 1.x filesystem.  It will of course
still be a Squashfs 2.0 filesystem but without fragments, and so it won't be
mountable on a Squashfs 1.x system.

The -always-use-fragments option tells mksquashfs to always generate
fragments for files irrespective of the file length.  By default only small
files less than the block size are packed into fragment blocks.  The ends of
files which do not fit fully into a block, are NOT by default packed into
fragments.  To illustrate this, a 100K file has an initial 64K block and a 36K
remainder.  This 36K remainder is not packed into a fragment by default.  This
is because to do so leads to a 10 - 20% drop in sequential I/O performance, as a
disk head seek is needed to seek to the initial file data and another disk seek
is need to seek to the fragment block.  Specify this option if you want file
remainders to be packed into fragment blocks.  Doing so may increase the
compression obtained BUT at the expense of I/O speed.

The -no-duplicates option tells mksquashfs to not check the files being
added to the filesystem for duplicates.  This can result in quicker filesystem
generation and appending although obviously compression will suffer badly if
there is a lot of duplicate files.

The -b option allows the block size to be selected, this can be either
4096, 8192, 16384, 32768 or 65536 bytes.

3.3 Specifying the UIDs/GIDs used in the filesystem

By default files in the generated filesystem inherit the UID and GID ownership
of the original file.  However,  mksquashfs provides a number of options which
can be used to override the ownership.

The options -all-root and -root-owned (both do exactly the same thing) force all
file uids/gids in the generated Squashfs filesystem to be root.  This allows
root owned filesystems to be built without root access on the host machine.

The "-force-uid uid"  option forces all files in the generated Squashfs
filesystem to be owned by the specified uid.  The uid can be specified either by
name (i.e. "root") or by number.

The "-force-gid gid" option forces all files in the generated Squashfs
filesystem to be group owned by the specified gid.  The gid can be specified
either by name (i.e. "root") or by number.

3.4 Excluding files from the filesystem

The -e and -ef options allow files/directories to be specified which are
excluded from the output filesystem.  The -e option takes the exclude
files/directories from the command line, the -ef option takes the
exlude files/directories from the specified exclude file, one file/directory
per line. If an exclude file/directory is absolute (i.e. prefixed with /, ../,
or ./) the entry is treated as absolute, however, if an exclude file/directory
is relative, it is treated as being relative to each of the sources in turn,

%mksquashfs /tmp/source1 source2  output_fs -e ex1 /tmp/source1/ex2 out/ex3

Will generate exclude files /tmp/source1/ex2, /tmp/source1/ex1, source2/ex1,
/tmp/source1/out/ex3 and source2/out/ex3.

The -e and -ef exclude options are usefully used in archiving the entire
filesystem, where it is wished to avoid archiving /proc, and the filesystem
being generated, i.e.

%mksquashfs / /tmp/root.sqsh -e proc /tmp/root.sqsh

Multiple -ef options can be specified on the command line, and the -ef
option can be used in conjuction with the -e option.

3.5 Appending to squashfs filesystems

Running squashfs with the destination directory containing an existing
filesystem will add the source items to the existing filesystem.  By default,
the source items are added to the existing root directory.

To make this clear... An existing filesystem "image" contains root entries
"old1", and "old2".  Source directory "/home/phillip/test" contains  "file1",
"file2" and "dir1".

example 1:

%mksquashfs /home/phillip/test image

Will create a new "image" with root entries "old1", "old2", "file1", "file2" and

example 2:

%mksquashfs /home/phillip/test image -keep-as-directory

Will create a new "image" with root entries "old1", "old2", and "test".
As shown in the previous section, for single source directories
'-keep-as-directory' adds the source directory rather than the
contents of the directory.

example 3:

%mksquashfs /home/phillip/test image -keep-as-directory -root-becomes

Will create a new "image" with root entries "original-root", and "test".  The
'-root-becomes' option specifies that the original root becomes a subdirectory
in the new root, with the specified name.

The append option with file duplicate detection, means squashfs can be
used as a simple versioning archiving filesystem. A squashfs filesystem can
be created with for example the linux-2.4.19 source.  Appending the linux-2.4.20
source will create a filesystem with the two source trees, but only the
changed files will take extra room, the unchanged files will be detected as

3.6 Miscellaneous options

The -info option displays the files/directories as they are compressed and
added to the filesystem.  The original uncompressed size of each file
is printed, along with DUPLICATE if the file is a duplicate of a
file in the filesystem.

The -le and -be options can be used to force mksquashfs to generate a little
endian or big endian filesystem.  Normally mksquashfs will generate a
filesystem in the host byte order.  Squashfs, for portability, will
mount different ordered filesystems (i.e. it can mount big endian filesystems
running on a little endian machine), but these options can be used for
greater optimisation.

The -nopad option informs mksquashfs to not pad the filesystem to a 4K multiple.
This is performed by default to enable the output filesystem file to be mounted
by loopback, which requires files to be a 4K multiple.  If the filesystem is
being written to a block device, or is to be stored in a bootimage, the extra
pad bytes are not needed.


Brief filesystem design notes follow for the original 1.x filesystem
layout.  A description of the 2.0 filesystem layout will be written sometime!

A squashfs filesystem consists of five parts, packed together on a byte

	|  superblock 	|
	|     data	|
	|    blocks	|
	|    inodes	|
	|   directories	|
	|    uid/gid	|
	|  lookup table	|

Compressed data blocks are written to the filesystem as files are read from
the source directory, and checked for duplicates.  Once all file data has been
written the completed inode, directory and uid/gid lookup tables are written.

4.1 Metadata

Metadata (inodes and directories) are compressed in 8Kbyte blocks.  Each
compressed block is prefixed by a two byte length, the top bit is set if the
block is uncompressed.  A block will be uncompressed if the -noI option is set,
or if the compressed block was larger than the uncompressed block.

Inodes are packed into the metadata blocks, and are not aligned to block
boundaries, therefore inodes overlap compressed blocks.  An inode is
identified by a two field tuple <start address of compressed block : offset
into de-compressed block>.

Inode contents vary depending on the file type.  The base inode consists of:

	base inode:
		Inode type
		uid index
		gid index

The inode type is 4 bits in size, and the mode is 12 bits.

The uid and gid indexes are 4 bits in length.  Ordinarily, this will allow 16
unique indexes into the uid table.  To minimise overhead, the uid index is
used in conjunction with the spare bit in the file type to form a 48 entry
index as follows:

	inode type 1 - 5: uid index = uid
	inode type 5 -10: uid index = 16 + uid
	inode type 11 - 15: uid index = 32 + uid

In this way 48 unique uids are supported using 4 bits, minimising data inode
overhead.  The 4 bit gid index is used to index into a 15 entry gid table.
Gid index 15 is used to indicate that the gid is the same as the uid.
This prevents the 15 entry gid table filling up with the common case where
the uid/gid is the same.

The data contents of symbolic links are stored immediately after the symbolic
link inode, inside the inode table.  This allows the normally small symbolic
link to be compressed as part of the inode table, achieving much greater
compression than if the symbolic link was compressed individually.

Similarly, the block index for regular files is stored immediately after the
regular file inode.  The block index is a list of block lengths (two bytes
each), rather than block addresses, saving two bytes per block.  The block
address for a given block is computed by the summation of the previous
block lengths.  This takes advantage of the fact that the blocks making up a
file are stored contiguously in the filesystem.  The top bit of each block
length is set if the block is uncompressed, either because the -noD option is
set, or if the compressed block was larger than the uncompressed block.

4.2 Directories

Like inodes, directories are packed into the metadata blocks, and are not
aligned on block boundaries, therefore directories can overlap compressed
blocks.  A directory is, again, identified by a two field tuple
<start address of compressed block containing directory start : offset
into de-compressed block>.

Directories are organised in a slightly complex way, and are not simply
a list of file names and inode tuples.  The organisation takes advantage of the
observation that in most cases, the inodes of the files in the directory
will be in the same compressed metadata block, and therefore, the
inode tuples will have the same start block.

Directories are therefore organised in a two level list, a directory
header containing the shared start block value, and a sequence of
directory entries, each of which share the shared start block.  A
new directory header is written once/if the inode start block
changes.  The directory header/directory entry list is repeated as many times
as necessary.  The organisation is as follows:

		count (8 bits)
		inode start block (24 bits)
		directory entry: * count
			inode offset (13 bits)
			inode type (3 bits)
			filename size (8 bits)
This organisation saves on average 3 bytes per filename.

4.3 File data

File data is compressed on a block by block basis and written to the
filesystem.  The filesystem supports up to 32K blocks, which achieves
greater compression ratios than the Linux 4K page size.

The disadvantage with using greater than 4K blocks (and the reason why
most filesystems do not), is that the VFS reads data in 4K pages.
The filesystem reads and decompresses a larger block containing that page
(e.g. 32K).  However, only 4K can be returned to the VFS, resulting in a
very inefficient filesystem, as 28K must be thrown away.   Squashfs,
solves this problem by explicitly pushing the extra pages into the page


Squashfs was written by Phillip Lougher, email phillip@lougher.demon.co.uk,
in Chepstow, Wales, UK.   If you like the program, or have any problems,
then please email me, as it's nice to get feedback!