RELEASE-READMEs/README-2.0 - pub/scm/fs/squashfs/squashfs-tools - Git at Google

 NOTE:  This the original README for version 2.0.  It is retained as it
 contains information about the fragment design.  A description of the new 2.0
 mksquashfs options has been added to the main README file, and that
 file should now be consulted for these.

 	SQUASHFS 2.0 - A squashed read-only filesystem for Linux

 	Copyright 2004 Phillip Lougher (plougher@users.sourceforge.net)

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

 Welcome to the final release of Squashfs version 2.0!  A lot of changes to the
 filesystem have been made under the bonnet (hood).  Squashfs 2.0 uses fragment
 blocks and larger blocks (64K) to improve compression ratio by about 5 - 20%
 over Squashfs 1.0 depending on the files being compressed.  Using fragment
 blocks allows Squashfs 2.0 to achieve better compression than cloop and similar
 compression to tgz files while retaining the I/O efficiency of a compressed
 filesystem.

 Detailed changes:

 1. Squashfs 2.0 has added the concept of fragment blocks (see later discussion).
    Files smaller than the file block size (64K in Squashfs 2.0) and optionally
    the remainder of files that do not fit fully into a block (i.e. the last 32K
    in a 96K file) are packed into shared fragments and compressed together.
    This achieves on average 5 - 20% better compression than Squashfs 1.x.

 2. The maximum block size has been increased to 64K.

 3. The maximum number of UIDs has been increased to 256 (from 48 in 1.x).

 4. The maximum number of GIDs has been increased to 256 (from 15 in 1.x).

 5. New mksquashfs -all-root, -root-owned, -force-uid, and -force-gid
    options.  These allow the uids/gids of files in the generated
    filesystem to be specified, overriding the uids/gids in the
    source filesystem.

 6. Initrds are now supported for kernels 2.6.x.

 7. Removal of sleep_on() function call in 2.6.x patch, to allow Squashfs
    to work on the Fedora rc2 kernel.

 8. AMD64, check-data and gid bug fixes.

 9. Numerous small bug fixes have been made.

 10. New patch for Linux 2.6.7.


 New Squashfs 2.0 options
 ------------------------

 -noF or -noFragmentCompression

 	Do not compress the fragments.  Added for compatibility with noI and
 	noD, probably not that useful.

 -no-fragments

 	Do not use 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.

 -always-use-fragments

 	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.

 -no-duplicates

 	Do not detect duplicate files.

 -all-root
 -root-owned

 	These options (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.

 -force-uid uid

 	This 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.

 -force-gid gid

 	This 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.


 Compression improvements example
 --------------------------------

 The following is the compression results obtained compressing the 2.6.6
 linux kernel source using CRAMFS, Cloop (with iso filesystem), Squashfs 1.3 and
 Squashfs 2.0 (results generated using big-endian filesystems).

 In decreasing order of size:

 	CRAMFS		62791680 bytes (59.9M)
 	Squashfs 1.x	51351552 bytes (48.9M)
 	Cloop		46118681 bytes (44.0M)
 	Squashfs 2.0	45604854 bytes (43.5M)


 The Squashfs 1.x filesystem is 12.6% larger than the new 2.0 filesystem.
 The cloop filesystem is 1.1% larger than the Squashfs 2.0 filesystem.


 Fragment blocks in Squashfs 2.0
 -------------------------------

 Squashfs like all other compressed filesystems compresses files individually
 on a block by block basis.  This is performed to allow mounting and
 de-compression of files on a block by block basis without requiring the entire
 filesystem to be decompressed.  This is in contrast to data-based compression
 schemes which compress without understanding the underlying filesystem (i.e.
 cloop and tgz files) and which, therefore, do not compress files individually.
 Each approach has advantages and disadvantages, data-based systems have better
 compression because compression is always performed at the maximum block size
 (64K in cloop) irrespective of the size of each file (which could be less than
 the block size).  Compressed filesystems tend to be faster at I/O because
 they understand the filesystem and therefore employ better caching stategies
 and read less un-needed data from the filesystem.

 Fragment blocks in Squashfs 2.0 solves this problem by packing files (and
 optionally the ends of files) which are smaller than the block size into
 shared blocks, which are compressed together.  For example five files each of
 10K will be packed into one shared fragment of 50K and compressed together,
 rather than being compressed in five 10K blocks.

 This scheme produces a hybrid filesystem, retaining the I/O efficiency
 of a compressed filesystem, while obtaining the compression efficiency
 of data-based schemes by compressing small files together.


 Squashfs 1.x and Squashfs 2.0 compatibility
 -------------------------------------------

 Appending to Squashfs 1.x filesystems is not supported.  If you wish to append
 to 1.x filesystems, then either use the original mksquashfs, or convert them
 to Squashfs 2.0 by mounting the filesystem and running the 2.0 mksquashfs
 on the mounted filesystem.

 Mounting Squashfs 1.x filesystems IS supported by the 2.0 kernel patch.
	NOTE: This the original README for version 2.0. It is retained as it
	contains information about the fragment design. A description of the new 2.0
	mksquashfs options has been added to the main README file, and that
	file should now be consulted for these.

	SQUASHFS 2.0 - A squashed read-only filesystem for Linux

	Copyright 2004 Phillip Lougher (plougher@users.sourceforge.net)

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

	Welcome to the final release of Squashfs version 2.0! A lot of changes to the
	filesystem have been made under the bonnet (hood). Squashfs 2.0 uses fragment
	blocks and larger blocks (64K) to improve compression ratio by about 5 - 20%
	over Squashfs 1.0 depending on the files being compressed. Using fragment
	blocks allows Squashfs 2.0 to achieve better compression than cloop and similar
	compression to tgz files while retaining the I/O efficiency of a compressed
	filesystem.

	Detailed changes:

	1. Squashfs 2.0 has added the concept of fragment blocks (see later discussion).
	Files smaller than the file block size (64K in Squashfs 2.0) and optionally
	the remainder of files that do not fit fully into a block (i.e. the last 32K
	in a 96K file) are packed into shared fragments and compressed together.
	This achieves on average 5 - 20% better compression than Squashfs 1.x.

	2. The maximum block size has been increased to 64K.

	3. The maximum number of UIDs has been increased to 256 (from 48 in 1.x).

	4. The maximum number of GIDs has been increased to 256 (from 15 in 1.x).

	5. New mksquashfs -all-root, -root-owned, -force-uid, and -force-gid
	options. These allow the uids/gids of files in the generated
	filesystem to be specified, overriding the uids/gids in the
	source filesystem.

	6. Initrds are now supported for kernels 2.6.x.

	7. Removal of sleep_on() function call in 2.6.x patch, to allow Squashfs
	to work on the Fedora rc2 kernel.

	8. AMD64, check-data and gid bug fixes.

	9. Numerous small bug fixes have been made.

	10. New patch for Linux 2.6.7.


	New Squashfs 2.0 options
	------------------------

	-noF or -noFragmentCompression

	Do not compress the fragments. Added for compatibility with noI and
	noD, probably not that useful.

	-no-fragments

	Do not use 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.

	-always-use-fragments

	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.

	-no-duplicates

	Do not detect duplicate files.

	-all-root
	-root-owned

	These options (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.

	-force-uid uid

	This 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.

	-force-gid gid

	This 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.


	Compression improvements example
	--------------------------------

	The following is the compression results obtained compressing the 2.6.6
	linux kernel source using CRAMFS, Cloop (with iso filesystem), Squashfs 1.3 and
	Squashfs 2.0 (results generated using big-endian filesystems).

	In decreasing order of size:

	CRAMFS 62791680 bytes (59.9M)
	Squashfs 1.x 51351552 bytes (48.9M)
	Cloop 46118681 bytes (44.0M)
	Squashfs 2.0 45604854 bytes (43.5M)


	The Squashfs 1.x filesystem is 12.6% larger than the new 2.0 filesystem.
	The cloop filesystem is 1.1% larger than the Squashfs 2.0 filesystem.


	Fragment blocks in Squashfs 2.0
	-------------------------------

	Squashfs like all other compressed filesystems compresses files individually
	on a block by block basis. This is performed to allow mounting and
	de-compression of files on a block by block basis without requiring the entire
	filesystem to be decompressed. This is in contrast to data-based compression
	schemes which compress without understanding the underlying filesystem (i.e.
	cloop and tgz files) and which, therefore, do not compress files individually.
	Each approach has advantages and disadvantages, data-based systems have better
	compression because compression is always performed at the maximum block size
	(64K in cloop) irrespective of the size of each file (which could be less than
	the block size). Compressed filesystems tend to be faster at I/O because
	they understand the filesystem and therefore employ better caching stategies
	and read less un-needed data from the filesystem.

	Fragment blocks in Squashfs 2.0 solves this problem by packing files (and
	optionally the ends of files) which are smaller than the block size into
	shared blocks, which are compressed together. For example five files each of
	10K will be packed into one shared fragment of 50K and compressed together,
	rather than being compressed in five 10K blocks.

	This scheme produces a hybrid filesystem, retaining the I/O efficiency
	of a compressed filesystem, while obtaining the compression efficiency
	of data-based schemes by compressing small files together.


	Squashfs 1.x and Squashfs 2.0 compatibility
	-------------------------------------------

	Appending to Squashfs 1.x filesystems is not supported. If you wish to append
	to 1.x filesystems, then either use the original mksquashfs, or convert them
	to Squashfs 2.0 by mounting the filesystem and running the 2.0 mksquashfs
	on the mounted filesystem.

	Mounting Squashfs 1.x filesystems IS supported by the 2.0 kernel patch.