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183 lines
7.2 KiB
183 lines
7.2 KiB
SQUASHFS 4.3 - A squashed read-only filesystem for Linux
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Copyright 2002-2014 Phillip Lougher <phillip@lougher.demon.co.uk>
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Released under the GPL licence (version 2 or later).
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Welcome to Squashfs 4.3. This is the first release in over 3 years, and
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there are substantial improvements to stability, new compression options
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and compressors, speed optimisations, and new options for Mksquashfs/Unsquashfs.
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This is a tools only release, support for Squashfs filesystems is
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in mainline (2.6.29 and later).
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Changes in Squashfs-tools 4.3
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-----------------------------
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1. Stability improvements. Better checking of user input for out of
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range/invalid values. Better handling of corrupted Squashfs filesystems
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(Mksquashfs append mode, and Unsquashfs). Better handling of buffer
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overflow/underflow.
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2. GZIP compressor now supports compression options, allowing different
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compression levels to be used.
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3. Rewritten LZO compressor with compression options, allowing different
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LZO algorithms and different compression levels to be used.
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4. New LZ4 compressor (note not yet in mainline kernel)
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5. Better default memory usage for Mksquashfs. Mksquashfs by default now
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uses 25% of physical memory.
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6. Duplicate checking in Mksquashfs further optimised. With certain
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"problem filesystems" greater than 2x performance improvement.
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Filesystems with a lot of duplicates should see at least 10-20% speed
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improvement.
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7. The -stat option in Unsquashfs now displays the compression options
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used to generate the original filesystem. Previously -stat only displayed
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the compression algorithm used.
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8. The file being compressed/uncompressed in Mksquashfs/Unsquashfs is now
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displayed if CTRL-\ (SIGQUIT from keyboard) typed.
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9. The status of the internal queues/caches in Mksquashfs/Unsquashfs is
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now displayed if CTRL-\ (SIGQUIT from keyboard) is typed twice within
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one second. Normally only useful for "power users", but it can be
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used to discover if there's any bottlenecks affecting performance
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(the bottleneck will normally be the compressors/fragment compressors).
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10. Miscellaneous new options for Mksquashfs/Unsquashfs to fine tune behaviour.
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11. Fixes for CVE-2012-4024 and CVE-2012-4025.
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Compatiblity
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------------
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Mksquashfs 4.3 generates 4.0 filesystems. These filesystems are fully
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compatible/interchangable with filesystems generated by Mksquashfs 4.0 and are
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mountable on 2.6.29 and later kernels.
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Compressors
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-----------
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New compression options and compressors are now supported.
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The new options and compressors are:
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1. gzip
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-Xcompression-level <compression-level>
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<compression-level> should be 1 .. 9 (default 9)
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-Xwindow-size <window-size>
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<window-size> should be 8 .. 15 (default 15)
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-Xstrategy strategy1,strategy2,...,strategyN
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Compress using strategy1,strategy2,...,strategyN in turn
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and choose the best compression.
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Available strategies: default, filtered, huffman_only,
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run_length_encoded and fixed
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2. lzo
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-Xalgorithm <algorithm>
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Where <algorithm> is one of:
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lzo1x_1
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lzo1x_1_11
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lzo1x_1_12
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lzo1x_1_15
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lzo1x_999 (default)
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-Xcompression-level <compression-level>
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<compression-level> should be 1 .. 9 (default 8)
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Only applies to lzo1x_999 algorithm
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3. lz4
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-Xhc
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Compress using LZ4 High Compression
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The compression specific options are, obviously, specific to the compressor
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in question, and you should read the compressor documentation and check
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their web sites to understand their behaviour.
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In general the defaults used by Mksquashfs for each compressor are optimised
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to give the best performance for each compressor, where what constitutes
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best depends on the compressor. For gzip/xz best means highest compression
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(trying multiple filters/strategies can improve compression, but this is
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extremely expensive computationally, and hence, not suitable for the defaults),
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for LZO/LZ4 best means a tradeoff between compression and (de)-compression
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overhead (LZO/LZ4 by definition are intended for weaker processors).
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New Mksquashfs options
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----------------------
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1. -mem <size>
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Set the amount of memory used by Mksquashfs to <size> bytes. G/M and K
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post-fixes are supported.
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By default Mksquashfs uses 25% of the physical memory. Increasing
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this with the -mem option can increase performance (note it does not have
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any effect on compression). Reducing it can prevent thrashing if the
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system is busy and there is not 25% of physical memory free (again, note
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it does not have any effect on compression).
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2. -exit-on-error
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By default Mksquashfs treats certain errors as benign, if these
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errors occur Mksquashfs prints the error on the console but continues.
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These errors are typically failure to read a file from the source filesystem.
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This is deliberate, in many cases users prefer Mksquashfs to flag
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the error but continue rather than abort what may be hours of compression.
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But there are times where failure to read any file is considered critical,
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and users (especially in the case of automated scripts where the
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errors output to the console may be missed) prefer Mksquashfs to exit.
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The new -exit-on-error option can be used in this scenario. This option
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makes Mksquashfs treat all benign errors as fatal.
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3. -progress
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By default if -info is specified, the progress bar is disabled as it gets
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in the way. Occasionally you might want the progress bar enabled whilst
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-info is enabled. This option forces Mksquashfs to output the progress
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bar when -info is specified.
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4. -Xhelp
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Display the usage text for the currently selected compressor.
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New Unsquashfs options
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----------------------
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1. -u[ser-xattrs]
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Only write user xattrs. This forces Unsquashfs to ignore system xattrs.
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This is useful when Unsquashing a filesystem as a non-root user, and the
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filesystem contains system xattrs which are only writable by root.
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Major bugs fixed
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----------------
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1. If Mksquashfs ran out of space in the destination filesystem, this
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would not cause Mksquashfs to immediately abort, and Mksquashfs would
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continue to process the source filesystem. Mksquashfs now immediately
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aborts on out of space in the destination filesystem.
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2. Unsquashfs ignored the maximum number of open files limit, and if that
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was lower than the default limit for Linux, it would run out of file
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descriptors. Unsquashfs now limits the number of open files to the
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limit currently in force (e.g. specified by setrlimit).
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3. If huge numbers of dynamic pseudo files were specified, Mksquashfs
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could exceed the maximum number of open files limit. This was because
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Mksquashfs created all the dynamic file processes up front before
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commencing source filesystem reading and compression. Mksquashfs
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now creates the dynamic file processes on demand whilst reading
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and compressing the source filesystem, thus limiting the number of
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dynamic pseudo file processes in existence at any one time.
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4. When outputting Unsquashfs used to set the permissions of directories
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as it recursively descended. This in hindsight had an obvious oversight,
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if a directory had only read permission (or was otherwise restricted), then
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Unsquashfs would fail to write its contents when descending into it. Fixed
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by setting directory permissions as Unsquashfs recursively unwinds.
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