path: root/Documentation/xz.txt
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authorLasse Collin <>2011-01-12 17:01:22 -0800
committerLinus Torvalds <>2011-01-13 08:03:24 -0800
commit24fa0402a9b6a537e87e38341e78b7da86486846 (patch)
tree06adb32802cf8a3491dff1f4e5cad464c676040a /Documentation/xz.txt
parentfb7fa589fd3ecc212fabd7867a4ecc3b175260c1 (diff)
decompressors: add XZ decompressor module
In userspace, the .lzma format has become mostly a legacy file format that got superseded by the .xz format. Similarly, LZMA Utils was superseded by XZ Utils. These patches add support for XZ decompression into the kernel. Most of the code is as is from XZ Embedded <>. It was written for the Linux kernel but is usable in other projects too. Advantages of XZ over the current LZMA code in the kernel: - Nice API that can be used by other kernel modules; it's not limited to kernel, initramfs, and initrd decompression. - Integrity check support (CRC32) - BCJ filters improve compression of executable code on certain architectures. These together with LZMA2 can produce a few percent smaller kernel or Squashfs images than plain LZMA without making the decompression slower. This patch: Add the main decompression code (xz_dec), testing module (xz_dec_test), wrapper script ( for the xz command line tool, and documentation. The xz_dec module is enough to have a usable XZ decompressor e.g. for Squashfs. Signed-off-by: Lasse Collin <> Cc: "H. Peter Anvin" <> Cc: Alain Knaff <> Cc: Albin Tonnerre <> Cc: Phillip Lougher <> Signed-off-by: Andrew Morton <> Signed-off-by: Linus Torvalds <>
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+XZ data compression in Linux
+ 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
+ <>, 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
+ <>.
+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/ 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 <> to get the
+ latest code.
+ Report bugs to <> 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.