xz.txt: standardize document format
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>
This commit is contained in:
parent
05a07392c3
commit
29c8c4ac95
@ -1,121 +1,127 @@
|
||||
|
||||
============================
|
||||
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.
|
||||
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.
|
||||
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/>.
|
||||
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 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.
|
||||
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.
|
||||
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.
|
||||
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.
|
||||
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.
|
||||
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.
|
||||
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:
|
||||
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
|
||||
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.
|
||||
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.
|
||||
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.
|
||||
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.
|
||||
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.
|
||||
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.
|
||||
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.
|
||||
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.
|
||||
|
Loading…
Reference in New Issue
Block a user