SQUASHFS 4.0 - A squashed read-only filesystem for Linux
Copyright 2002-2009 Phillip Lougher <phillip@lougher.demon.co.uk>
Released under the GPL licence (version 2 or later).
Welcome to Squashfs version 4.0. Please read the README-4.0 and CHANGES files
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 1Mbytes (default block size 128K).
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
needed.
1. SQUASHFS OVERVIEW
--------------------
1. Data, inodes and directories are compressed.
2. Squashfs stores full uid/gids (32 bits), and file creation time.
3. In theory files up to 2^64 bytes are supported. In theory filesystems can
be up to 2^64 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 1Mbyte (the default size is 128K).
Using 128K 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.
2. USING SQUASHFS
-----------------
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. MKSQUASHFS
-------------
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
dirs/files]
Options are
-version print version, licence and copyright message
-recover <name> recover filesystem data using recovery file <name>
-no-recovery don't generate a recovery file
-info print files written to filesystem
-no-exports don't make the filesystem exportable via NFS
-no-progress don't display the progress bar
-no-sparse don't detect sparse files
-b <block_size> set data block to <block_size>. Default 131072 bytes
-processors <number> Use <number> processors. By default will use number of
processors available
-read-queue <size> Set input queue to <size> Mbytes. Default 64 Mbytes
-write-queue <size> Set output queue to <size> Mbytes. Default 512 Mbytes
-fragment-queue <size> Set fagment queue to <size> Mbytes. Default 64 Mbytes
-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
-nopad do not pad filesystem to a multiple of 4K
-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
-wildcards Allow extended shell wildcards (globbing) to be used in
exclude dirs/files
-regex Allow POSIX regular expressions to be used in exclude
dirs/files
-p <pseudo-definition> Add pseudo file definition
-pf <pseudo-file> Add list of pseudo file definitions
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
filesystems.
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
present.
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 3.1 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, both "K" and "M" postfixes
are supported, this can be either 4K, 8K, 16K, 32K, 64K, 128K, 256K, 512K or
1M 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.
Two styles of exclude file matching are supported: basic exclude matching, and
extended wildcard matching. Basic exclude matching is a legacy feature
retained for backwards compatibility with earlier versions of Mksquashfs.
Extended wildcard matching should be used in preference.
3.4.1 Basic exclude matching
Each exclude file is treated as an exact match of a file/directory in
the source directories. 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, i.e.
%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.
3.4.2 Extended exclude file handling
Extended exclude file matching treats each exclude file as a wildcard or
regex expression. To enable wildcard matching specify the -wildcards
option, and to enable regex matching specify the -regex option. In most
cases the -wildcards option should be used rather than -regex because wildcard
matching behaviour is significantly easier to understand!
In addition to wildcards/regex expressions, exclude files can be "anchored" or
"non-anchored". An anchored exclude is one which matches from the root of the
directory and nowhere else, a non-anchored exclude matches anywhere. For
example given the directory hierarchy "a/b/c/a/b", the anchored exclude
"a/b" will match "a/b" at the root of the directory hierarchy, but
it will not match the "/a/b" sub-directory within directory "c", whereas a
non-anchored exclude would.
A couple of examples should make this clearer.
Anchored excludes
1. mksquashfs example image.sqsh -wildcards -e 'test/*.gz'
Exclude all files matching "*.gz" in the top level directory "test".
2. mksquashfs example image.sqsh -wildcards -e '*/[Tt]est/example*'
Exclude all files beginning with "example" inside directories called
"Test" or "test", that occur inside any top level directory.
Using extended wildcards, negative matching is also possible.
3. mksquashfs example image.sqsh -wildcards -e 'test/!(*data*).gz'
Exclude all files matching "*.gz" in top level directory "test",
except those with "data" in the name.
Non-anchored excludes
By default excludes match from the top level directory, but it is
often useful to exclude a file matching anywhere in the source directories.
For this non-anchored excludes can be used, specified by pre-fixing the
exclude with "...".
Examples:
1. mksquashfs example image.sqsh -wildcards -e '... *.gz'
Exclude files matching "*.gz" anywhere in the source directories.
For example this will match "example.gz", "test/example.gz", and
"test/test/example.gz".
2. mksquashfs example image.sqsh -wildcards -e '... [Tt]est/*.gz'
Exclude files matching "*.gz" inside directories called "Test" or
"test" that occur anywhere in the source directories.
Again, using extended wildcards, negative matching is also possible.
3. mksquashfs example image.sqsh -wildcards -e '... !(*data*).gz'
Exclude all files matching "*.gz" anywhere in the source directories,
except those with "data" in the name.
3.4.3 Exclude files summary
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
"dir1"
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
original-root
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
duplicates.
3.6 Appending recovery file feature
-----------------------------------
Recovery files are created when appending to existing Squashfs
filesystems. This allows the original filesystem to be recovered
if Mksquashfs aborts unexpectedly (i.e. power failure).
The recovery files are called squashfs_recovery_xxx_yyy, where
"xxx" is the name of the filesystem being appended to, and "yyy" is a
number to guarantee filename uniqueness (the PID of the parent Mksquashfs
process).
Normally if Mksquashfs exits correctly the recovery file is deleted to
avoid cluttering the filesystem. If Mksquashfs aborts, the "-recover"
option can be used to recover the filesystem, giving the previously
created recovery file as a parameter, i.e.
mksquashfs dummy image.sqsh -recover squashfs_recovery_image.sqsh_1234
The writing of the recovery file can be disabled by specifying the
"-no-recovery" option.
3.7 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 -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.
4. UNSQUASHFS
-------------
Unsquashfs allows you to decompress and extract a Squashfs filesystem without
mounting it. It can extract the entire filesystem, or a specific
file or directory.
The Unsquashfs usage info is:
SYNTAX: unsquashfs [options] filesystem [directories or files to extract]
-v[ersion] print version, licence and copyright information
-d[est] <pathname> unsquash to <pathname>, default "squashfs-root"
-n[o-progress] don't display the progress bar
-p[rocessors] <number> use <number> processors. By default will use
number of processors available
-i[nfo] print files as they are unsquashed
-li[nfo] print files as they are unsquashed with file
attributes (like ls -l output)
-l[s] list filesystem, but don't unsquash
-ll[s] list filesystem with file attributes (like
ls -l output), but don't unsquash
-f[orce] if file already exists then overwrite
-s[tat] display filesystem superblock information
-e[f] <extract file> list of directories or files to extract.
One per line
-da[ta-queue] <size> Set data queue to <size> Mbytes. Default 256
Mbytes
-fr[ag-queue] <size> Set fagment queue to <size> Mbytes. Default 256
Mbytes
-r[egex] treat extract names as POSIX regular expressions
rather than use the default shell wildcard
expansion (globbing)
To extract a subset of the filesystem, the filenames or directory
trees that are to be extracted can be specified on the command line. The
files/directories should be specified using the full path to the
files/directories as they appear within the Squashfs filesystem. The
files/directories will also be extracted to those positions within the specified
destination directory.
The extract files can also be given in a file using the "-e[f]" option.
Similarly to Mksquashfs, wildcard matching is performed on the extract
files. Wildcard matching is enabled by default.
Examples:
1. unsquashfs image.sqsh 'test/*.gz'
Extract all files matching "*.gz" in the top level directory "test".
2. unsquashfs image.sqsh '[Tt]est/example*'
Extract all files beginning with "example" inside top level directories
called "Test" or "test".
Using extended wildcards, negative matching is also possible.
3. unsquashfs image.sqsh 'test/!(*data*).gz'
Extract all files matching "*.gz" in top level directory "test",
except those with "data" in the name.
4.1 Unsquashfs options
----------------------
The "-ls" option can be used to list the contents of a filesystem without
decompressing the filesystem data itself. The "-lls" option is similar
but it also displays file attributes (ls -l style output).
The "-info" option forces Unsquashfs to print each file as it is decompressed.
The -"linfo" is similar but it also displays file attributes.
The "-dest" option specifies the directory that is used to decompress
the filesystem data. If this option is not given then the filesystem is
decompressed to the directory "squashfs-root" in the current working directory.
The "-force" option forces Unsquashfs to output to the destination
directory even if files or directories already exist. This allows you
to update an existing directory tree, or to Unsquashfs to a partially
filled directory. Without the "-force" option, Unsquashfs will
refuse to overwrite any existing files, or to create any directories if they
already exist. This is done to protect data in case of mistakes, and
so the "-force" option should be used with caution.
The "-stat" option displays filesystem superblock information. This is
useful to discover the filesystem version, byte ordering, whether it has a NFS
export table, and what options were used to compress the filesystem, etc.
Unsquashfs can decompress all Squashfs filesystem versions, 1.x, 2.x and 3.x
filesystems.
5. FILESYSTEM LAYOUT
--------------------
Brief filesystem design notes follow for the original 1.x filesystem
layout. A description of the 2.x and 3.x filesystem layouts will be written
sometime!
A squashfs filesystem consists of five parts, packed together on a byte
alignment:
---------------
| 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.
5.1 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
Mode
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.
5.2 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:
directory_header:
count (8 bits)
inode start block (24 bits)
directory entry: * count
inode offset (13 bits)
inode type (3 bits)
filename size (8 bits)
filename
This organisation saves on average 3 bytes per filename.
5.3 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
cache.
6. AUTHOR INFO
--------------
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!