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