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