FreeBSD/Linux Kernel Cross Reference
sys/Documentation/xz.txt

     
     XZ data compression in Linux
     ============================
     
     Introduction
     
         XZ is a general purpose data compression format with high compression
         ratio and relatively fast decompression. The primary compression
         algorithm (filter) is LZMA2. Additional filters can be used to improve
        compression ratio even further. E.g. Branch/Call/Jump (BCJ) filters
        improve compression ratio of executable data.
    
        The XZ decompressor in Linux is called XZ Embedded. It supports
        the LZMA2 filter and optionally also BCJ filters. CRC32 is supported
        for integrity checking. The home page of XZ Embedded is at
        <http://tukaani.org/xz/embedded.html>, where you can find the
        latest version and also information about using the code outside
        the Linux kernel.
    
        For userspace, XZ Utils provide a zlib-like compression library
        and a gzip-like command line tool. XZ Utils can be downloaded from
        <http://tukaani.org/xz/>.
    
    XZ related components in the kernel
    
        The xz_dec module provides XZ decompressor with single-call (buffer
        to buffer) and multi-call (stateful) APIs. The usage of the xz_dec
        module is documented in include/linux/xz.h.
    
        The xz_dec_test module is for testing xz_dec. xz_dec_test is not
        useful unless you are hacking the XZ decompressor. xz_dec_test
        allocates a char device major dynamically to which one can write
        .xz files from userspace. The decompressed output is thrown away.
        Keep an eye on dmesg to see diagnostics printed by xz_dec_test.
        See the xz_dec_test source code for the details.
    
        For decompressing the kernel image, initramfs, and initrd, there
        is a wrapper function in lib/decompress_unxz.c. Its API is the
        same as in other decompress_*.c files, which is defined in
        include/linux/decompress/generic.h.
    
        scripts/xz_wrap.sh is a wrapper for the xz command line tool found
        from XZ Utils. The wrapper sets compression options to values suitable
        for compressing the kernel image.
    
        For kernel makefiles, two commands are provided for use with
        $(call if_needed). The kernel image should be compressed with
        $(call if_needed,xzkern) which will use a BCJ filter and a big LZMA2
        dictionary. It will also append a four-byte trailer containing the
        uncompressed size of the file, which is needed by the boot code.
        Other things should be compressed with $(call if_needed,xzmisc)
        which will use no BCJ filter and 1 MiB LZMA2 dictionary.
    
    Notes on compression options
    
        Since the XZ Embedded supports only streams with no integrity check or
        CRC32, make sure that you don't use some other integrity check type
        when encoding files that are supposed to be decoded by the kernel. With
        liblzma, you need to use either LZMA_CHECK_NONE or LZMA_CHECK_CRC32
        when encoding. With the xz command line tool, use --check=none or
        --check=crc32.
    
        Using CRC32 is strongly recommended unless there is some other layer
        which will verify the integrity of the uncompressed data anyway.
        Double checking the integrity would probably be waste of CPU cycles.
        Note that the headers will always have a CRC32 which will be validated
        by the decoder; you can only change the integrity check type (or
        disable it) for the actual uncompressed data.
    
        In userspace, LZMA2 is typically used with dictionary sizes of several
        megabytes. The decoder needs to have the dictionary in RAM, thus big
        dictionaries cannot be used for files that are intended to be decoded
        by the kernel. 1 MiB is probably the maximum reasonable dictionary
        size for in-kernel use (maybe more is OK for initramfs). The presets
        in XZ Utils may not be optimal when creating files for the kernel,
        so don't hesitate to use custom settings. Example:
    
            xz --check=crc32 --lzma2=dict=512KiB inputfile
    
        An exception to above dictionary size limitation is when the decoder
        is used in single-call mode. Decompressing the kernel itself is an
        example of this situation. In single-call mode, the memory usage
        doesn't depend on the dictionary size, and it is perfectly fine to
        use a big dictionary: for maximum compression, the dictionary should
        be at least as big as the uncompressed data itself.
    
    Future plans
    
        Creating a limited XZ encoder may be considered if people think it is
        useful. LZMA2 is slower to compress than e.g. Deflate or LZO even at
        the fastest settings, so it isn't clear if LZMA2 encoder is wanted
        into the kernel.
    
        Support for limited random-access reading is planned for the
        decompression code. I don't know if it could have any use in the
        kernel, but I know that it would be useful in some embedded projects
        outside the Linux kernel.
    
    Conformance to the .xz file format specification
   
       There are a couple of corner cases where things have been simplified
       at expense of detecting errors as early as possible. These should not
       matter in practice all, since they don't cause security issues. But
       it is good to know this if testing the code e.g. with the test files
       from XZ Utils.
   
   Reporting bugs
   
       Before reporting a bug, please check that it's not fixed already
       at upstream. See <http://tukaani.org/xz/embedded.html> to get the
       latest code.
   
       Report bugs to <lasse.collin@tukaani.org> or visit #tukaani on
       Freenode and talk to Larhzu. I don't actively read LKML or other
       kernel-related mailing lists, so if there's something I should know,
       you should email to me personally or use IRC.
   
       Don't bother Igor Pavlov with questions about the XZ implementation
       in the kernel or about XZ Utils. While these two implementations
       include essential code that is directly based on Igor Pavlov's code,
       these implementations aren't maintained nor supported by him.

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