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Based on kernel version 4.13.3. Page generated on 2017-09-23 13:56 EST.

1	============================
2	XZ data compression in Linux
3	============================
4	
5	Introduction
6	============
7	
8	XZ is a general purpose data compression format with high compression
9	ratio and relatively fast decompression. The primary compression
10	algorithm (filter) is LZMA2. Additional filters can be used to improve
11	compression ratio even further. E.g. Branch/Call/Jump (BCJ) filters
12	improve compression ratio of executable data.
13	
14	The XZ decompressor in Linux is called XZ Embedded. It supports
15	the LZMA2 filter and optionally also BCJ filters. CRC32 is supported
16	for integrity checking. The home page of XZ Embedded is at
17	<http://tukaani.org/xz/embedded.html>, where you can find the
18	latest version and also information about using the code outside
19	the Linux kernel.
20	
21	For userspace, XZ Utils provide a zlib-like compression library
22	and a gzip-like command line tool. XZ Utils can be downloaded from
23	<http://tukaani.org/xz/>.
24	
25	XZ related components in the kernel
26	===================================
27	
28	The xz_dec module provides XZ decompressor with single-call (buffer
29	to buffer) and multi-call (stateful) APIs. The usage of the xz_dec
30	module is documented in include/linux/xz.h.
31	
32	The xz_dec_test module is for testing xz_dec. xz_dec_test is not
33	useful unless you are hacking the XZ decompressor. xz_dec_test
34	allocates a char device major dynamically to which one can write
35	.xz files from userspace. The decompressed output is thrown away.
36	Keep an eye on dmesg to see diagnostics printed by xz_dec_test.
37	See the xz_dec_test source code for the details.
38	
39	For decompressing the kernel image, initramfs, and initrd, there
40	is a wrapper function in lib/decompress_unxz.c. Its API is the
41	same as in other decompress_*.c files, which is defined in
42	include/linux/decompress/generic.h.
43	
44	scripts/xz_wrap.sh is a wrapper for the xz command line tool found
45	from XZ Utils. The wrapper sets compression options to values suitable
46	for compressing the kernel image.
47	
48	For kernel makefiles, two commands are provided for use with
49	$(call if_needed). The kernel image should be compressed with
50	$(call if_needed,xzkern) which will use a BCJ filter and a big LZMA2
51	dictionary. It will also append a four-byte trailer containing the
52	uncompressed size of the file, which is needed by the boot code.
53	Other things should be compressed with $(call if_needed,xzmisc)
54	which will use no BCJ filter and 1 MiB LZMA2 dictionary.
55	
56	Notes on compression options
57	============================
58	
59	Since the XZ Embedded supports only streams with no integrity check or
60	CRC32, make sure that you don't use some other integrity check type
61	when encoding files that are supposed to be decoded by the kernel. With
62	liblzma, you need to use either LZMA_CHECK_NONE or LZMA_CHECK_CRC32
63	when encoding. With the xz command line tool, use --check=none or
64	--check=crc32.
65	
66	Using CRC32 is strongly recommended unless there is some other layer
67	which will verify the integrity of the uncompressed data anyway.
68	Double checking the integrity would probably be waste of CPU cycles.
69	Note that the headers will always have a CRC32 which will be validated
70	by the decoder; you can only change the integrity check type (or
71	disable it) for the actual uncompressed data.
72	
73	In userspace, LZMA2 is typically used with dictionary sizes of several
74	megabytes. The decoder needs to have the dictionary in RAM, thus big
75	dictionaries cannot be used for files that are intended to be decoded
76	by the kernel. 1 MiB is probably the maximum reasonable dictionary
77	size for in-kernel use (maybe more is OK for initramfs). The presets
78	in XZ Utils may not be optimal when creating files for the kernel,
79	so don't hesitate to use custom settings. Example::
80	
81		xz --check=crc32 --lzma2=dict=512KiB inputfile
82	
83	An exception to above dictionary size limitation is when the decoder
84	is used in single-call mode. Decompressing the kernel itself is an
85	example of this situation. In single-call mode, the memory usage
86	doesn't depend on the dictionary size, and it is perfectly fine to
87	use a big dictionary: for maximum compression, the dictionary should
88	be at least as big as the uncompressed data itself.
89	
90	Future plans
91	============
92	
93	Creating a limited XZ encoder may be considered if people think it is
94	useful. LZMA2 is slower to compress than e.g. Deflate or LZO even at
95	the fastest settings, so it isn't clear if LZMA2 encoder is wanted
96	into the kernel.
97	
98	Support for limited random-access reading is planned for the
99	decompression code. I don't know if it could have any use in the
100	kernel, but I know that it would be useful in some embedded projects
101	outside the Linux kernel.
102	
103	Conformance to the .xz file format specification
104	================================================
105	
106	There are a couple of corner cases where things have been simplified
107	at expense of detecting errors as early as possible. These should not
108	matter in practice all, since they don't cause security issues. But
109	it is good to know this if testing the code e.g. with the test files
110	from XZ Utils.
111	
112	Reporting bugs
113	==============
114	
115	Before reporting a bug, please check that it's not fixed already
116	at upstream. See <http://tukaani.org/xz/embedded.html> to get the
117	latest code.
118	
119	Report bugs to <lasse.collin@tukaani.org> or visit #tukaani on
120	Freenode and talk to Larhzu. I don't actively read LKML or other
121	kernel-related mailing lists, so if there's something I should know,
122	you should email to me personally or use IRC.
123	
124	Don't bother Igor Pavlov with questions about the XZ implementation
125	in the kernel or about XZ Utils. While these two implementations
126	include essential code that is directly based on Igor Pavlov's code,
127	these implementations aren't maintained nor supported by him.
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