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SQUASHFS/squashfs-tools-4.4/RELEASE-READMEs/README-2.0
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SQUASHFS/squashfs-tools-4.4/RELEASE-READMEs/README-2.0
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NOTE: This the original README for version 2.0. It is retained as it
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contains information about the fragment design. A description of the new 2.0
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mksquashfs options has been added to the main README file, and that
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file should now be consulted for these.
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SQUASHFS 2.0 - A squashed read-only filesystem for Linux
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Copyright 2004 Phillip Lougher (plougher@users.sourceforge.net)
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Released under the GPL licence (version 2 or later).
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Welcome to the final release of Squashfs version 2.0! A lot of changes to the
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filesystem have been made under the bonnet (hood). Squashfs 2.0 uses fragment
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blocks and larger blocks (64K) to improve compression ratio by about 5 - 20%
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over Squashfs 1.0 depending on the files being compressed. Using fragment
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blocks allows Squashfs 2.0 to achieve better compression than cloop and similar
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compression to tgz files while retaining the I/O efficiency of a compressed
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filesystem.
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Detailed changes:
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1. Squashfs 2.0 has added the concept of fragment blocks (see later discussion).
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Files smaller than the file block size (64K in Squashfs 2.0) and optionally
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the remainder of files that do not fit fully into a block (i.e. the last 32K
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in a 96K file) are packed into shared fragments and compressed together.
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This achieves on average 5 - 20% better compression than Squashfs 1.x.
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2. The maximum block size has been increased to 64K.
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3. The maximum number of UIDs has been increased to 256 (from 48 in 1.x).
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4. The maximum number of GIDs has been increased to 256 (from 15 in 1.x).
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5. New mksquashfs -all-root, -root-owned, -force-uid, and -force-gid
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options. These allow the uids/gids of files in the generated
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filesystem to be specified, overriding the uids/gids in the
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source filesystem.
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6. Initrds are now supported for kernels 2.6.x.
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7. Removal of sleep_on() function call in 2.6.x patch, to allow Squashfs
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to work on the Fedora rc2 kernel.
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8. AMD64, check-data and gid bug fixes.
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9. Numerous small bug fixes have been made.
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10. New patch for Linux 2.6.7.
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New Squashfs 2.0 options
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------------------------
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-noF or -noFragmentCompression
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Do not compress the fragments. Added for compatibility with noI and
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noD, probably not that useful.
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-no-fragments
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Do not use fragment blocks, and rather generate a filesystem
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similar to a Squashfs 1.x filesystem. It will of course still
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be a Squashfs 2.0 filesystem but without fragments, and so
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it won't be mountable on a Squashfs 1.x system.
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-always-use-fragments
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By default only small files less than the block size are packed into
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fragment blocks. The ends of files which do not fit fully into a block,
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are NOT by default packed into fragments. To illustrate this, a
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100K file has an initial 64K block and a 36K remainder. This
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36K remainder is not packed into a fragment by default. This is
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because to do so leads to a 10 - 20% drop in sequential I/O
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performance, as a disk head seek is needed to seek to the initial
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file data and another disk seek is need to seek to the fragment
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block.
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Specify this option if you want file remainders to be packed into
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fragment blocks. Doing so may increase the compression obtained
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BUT at the expense of I/O speed.
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-no-duplicates
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Do not detect duplicate files.
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-all-root
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-root-owned
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These options (both do exactly the same thing), force all file
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uids/gids in the generated Squashfs filesystem to be root.
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This allows root owned filesystems to be built without root access
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on the host machine.
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-force-uid uid
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This option forces all files in the generated Squashfs filesystem to
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be owned by the specified uid. The uid can be specified either by
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name (i.e. "root") or by number.
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-force-gid gid
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This option forces all files in the generated Squashfs filesystem to
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be group owned by the specified gid. The gid can be specified either by
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name (i.e. "root") or by number.
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Compression improvements example
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--------------------------------
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The following is the compression results obtained compressing the 2.6.6
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linux kernel source using CRAMFS, Cloop (with iso filesystem), Squashfs 1.3 and
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Squashfs 2.0 (results generated using big-endian filesystems).
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In decreasing order of size:
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CRAMFS 62791680 bytes (59.9M)
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Squashfs 1.x 51351552 bytes (48.9M)
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Cloop 46118681 bytes (44.0M)
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Squashfs 2.0 45604854 bytes (43.5M)
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The Squashfs 1.x filesystem is 12.6% larger than the new 2.0 filesystem.
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The cloop filesystem is 1.1% larger than the Squashfs 2.0 filesystem.
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Fragment blocks in Squashfs 2.0
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-------------------------------
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Squashfs like all other compressed filesystems compresses files individually
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on a block by block basis. This is performed to allow mounting and
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de-compression of files on a block by block basis without requiring the entire
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filesystem to be decompressed. This is in contrast to data-based compression
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schemes which compress without understanding the underlying filesystem (i.e.
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cloop and tgz files) and which, therefore, do not compress files individually.
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Each approach has advantages and disadvantages, data-based systems have better
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compression because compression is always performed at the maximum block size
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(64K in cloop) irrespective of the size of each file (which could be less than
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the block size). Compressed filesystems tend to be faster at I/O because
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they understand the filesystem and therefore employ better caching stategies
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and read less un-needed data from the filesystem.
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Fragment blocks in Squashfs 2.0 solves this problem by packing files (and
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optionally the ends of files) which are smaller than the block size into
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shared blocks, which are compressed together. For example five files each of
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10K will be packed into one shared fragment of 50K and compressed together,
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rather than being compressed in five 10K blocks.
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This scheme produces a hybrid filesystem, retaining the I/O efficiency
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of a compressed filesystem, while obtaining the compression efficiency
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of data-based schemes by compressing small files together.
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Squashfs 1.x and Squashfs 2.0 compatibility
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-------------------------------------------
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Appending to Squashfs 1.x filesystems is not supported. If you wish to append
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to 1.x filesystems, then either use the original mksquashfs, or convert them
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to Squashfs 2.0 by mounting the filesystem and running the 2.0 mksquashfs
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on the mounted filesystem.
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Mounting Squashfs 1.x filesystems IS supported by the 2.0 kernel patch.
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