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Linux/Documentation/xz.txt

  1 
  2 XZ data compression in Linux
  3 ============================
  4 
  5 Introduction
  6 
  7     XZ is a general purpose data compression format with high compression
  8     ratio and relatively fast decompression. The primary compression
  9     algorithm (filter) is LZMA2. Additional filters can be used to improve
 10     compression ratio even further. E.g. Branch/Call/Jump (BCJ) filters
 11     improve compression ratio of executable data.
 12 
 13     The XZ decompressor in Linux is called XZ Embedded. It supports
 14     the LZMA2 filter and optionally also BCJ filters. CRC32 is supported
 15     for integrity checking. The home page of XZ Embedded is at
 16     <http://tukaani.org/xz/embedded.html>, where you can find the
 17     latest version and also information about using the code outside
 18     the Linux kernel.
 19 
 20     For userspace, XZ Utils provide a zlib-like compression library
 21     and a gzip-like command line tool. XZ Utils can be downloaded from
 22     <http://tukaani.org/xz/>.
 23 
 24 XZ related components in the kernel
 25 
 26     The xz_dec module provides XZ decompressor with single-call (buffer
 27     to buffer) and multi-call (stateful) APIs. The usage of the xz_dec
 28     module is documented in include/linux/xz.h.
 29 
 30     The xz_dec_test module is for testing xz_dec. xz_dec_test is not
 31     useful unless you are hacking the XZ decompressor. xz_dec_test
 32     allocates a char device major dynamically to which one can write
 33     .xz files from userspace. The decompressed output is thrown away.
 34     Keep an eye on dmesg to see diagnostics printed by xz_dec_test.
 35     See the xz_dec_test source code for the details.
 36 
 37     For decompressing the kernel image, initramfs, and initrd, there
 38     is a wrapper function in lib/decompress_unxz.c. Its API is the
 39     same as in other decompress_*.c files, which is defined in
 40     include/linux/decompress/generic.h.
 41 
 42     scripts/xz_wrap.sh is a wrapper for the xz command line tool found
 43     from XZ Utils. The wrapper sets compression options to values suitable
 44     for compressing the kernel image.
 45 
 46     For kernel makefiles, two commands are provided for use with
 47     $(call if_needed). The kernel image should be compressed with
 48     $(call if_needed,xzkern) which will use a BCJ filter and a big LZMA2
 49     dictionary. It will also append a four-byte trailer containing the
 50     uncompressed size of the file, which is needed by the boot code.
 51     Other things should be compressed with $(call if_needed,xzmisc)
 52     which will use no BCJ filter and 1 MiB LZMA2 dictionary.
 53 
 54 Notes on compression options
 55 
 56     Since the XZ Embedded supports only streams with no integrity check or
 57     CRC32, make sure that you don't use some other integrity check type
 58     when encoding files that are supposed to be decoded by the kernel. With
 59     liblzma, you need to use either LZMA_CHECK_NONE or LZMA_CHECK_CRC32
 60     when encoding. With the xz command line tool, use --check=none or
 61     --check=crc32.
 62 
 63     Using CRC32 is strongly recommended unless there is some other layer
 64     which will verify the integrity of the uncompressed data anyway.
 65     Double checking the integrity would probably be waste of CPU cycles.
 66     Note that the headers will always have a CRC32 which will be validated
 67     by the decoder; you can only change the integrity check type (or
 68     disable it) for the actual uncompressed data.
 69 
 70     In userspace, LZMA2 is typically used with dictionary sizes of several
 71     megabytes. The decoder needs to have the dictionary in RAM, thus big
 72     dictionaries cannot be used for files that are intended to be decoded
 73     by the kernel. 1 MiB is probably the maximum reasonable dictionary
 74     size for in-kernel use (maybe more is OK for initramfs). The presets
 75     in XZ Utils may not be optimal when creating files for the kernel,
 76     so don't hesitate to use custom settings. Example:
 77 
 78         xz --check=crc32 --lzma2=dict=512KiB inputfile
 79 
 80     An exception to above dictionary size limitation is when the decoder
 81     is used in single-call mode. Decompressing the kernel itself is an
 82     example of this situation. In single-call mode, the memory usage
 83     doesn't depend on the dictionary size, and it is perfectly fine to
 84     use a big dictionary: for maximum compression, the dictionary should
 85     be at least as big as the uncompressed data itself.
 86 
 87 Future plans
 88 
 89     Creating a limited XZ encoder may be considered if people think it is
 90     useful. LZMA2 is slower to compress than e.g. Deflate or LZO even at
 91     the fastest settings, so it isn't clear if LZMA2 encoder is wanted
 92     into the kernel.
 93 
 94     Support for limited random-access reading is planned for the
 95     decompression code. I don't know if it could have any use in the
 96     kernel, but I know that it would be useful in some embedded projects
 97     outside the Linux kernel.
 98 
 99 Conformance to the .xz file format specification
100 
101     There are a couple of corner cases where things have been simplified
102     at expense of detecting errors as early as possible. These should not
103     matter in practice all, since they don't cause security issues. But
104     it is good to know this if testing the code e.g. with the test files
105     from XZ Utils.
106 
107 Reporting bugs
108 
109     Before reporting a bug, please check that it's not fixed already
110     at upstream. See <http://tukaani.org/xz/embedded.html> to get the
111     latest code.
112 
113     Report bugs to <lasse.collin@tukaani.org> or visit #tukaani on
114     Freenode and talk to Larhzu. I don't actively read LKML or other
115     kernel-related mailing lists, so if there's something I should know,
116     you should email to me personally or use IRC.
117 
118     Don't bother Igor Pavlov with questions about the XZ implementation
119     in the kernel or about XZ Utils. While these two implementations
120     include essential code that is directly based on Igor Pavlov's code,
121     these implementations aren't maintained nor supported by him.

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