mirror of
https://github.com/torvalds/linux.git
synced 2024-12-16 16:12:52 +00:00
29c8c4ac95
Each text file under Documentation follows a different format. Some doesn't even have titles! Change its representation to follow the adopted standard, using ReST markups for it to be parseable by Sphinx: - Use marks for titles; - Adjust indentation. Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com> Signed-off-by: Jonathan Corbet <corbet@lwn.net>
128 lines
5.5 KiB
Plaintext
128 lines
5.5 KiB
Plaintext
============================
|
|
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.
|