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2 XZ Utils FAQ
3 ============
5 Q: What do the letters XZ mean?
7 A: Nothing. They are just two letters, which come from the file format
8 suffix .xz. The .xz suffix was selected, because it seemed to be
9 pretty much unused. It has no deeper meaning.
12 Q: What are LZMA and LZMA2?
14 A: LZMA stands for Lempel-Ziv-Markov chain-Algorithm. It is the name
15 of the compression algorithm designed by Igor Pavlov for 7-Zip.
16 LZMA is based on LZ77 and range encoding.
18 LZMA2 is an updated version of the original LZMA to fix a couple of
19 practical issues. In context of XZ Utils, LZMA is called LZMA1 to
20 emphasize that LZMA is not the same thing as LZMA2. LZMA2 is the
21 primary compression algorithm in the .xz file format.
24 Q: There are many LZMA related projects. How does XZ Utils relate to them?
26 A: 7-Zip and LZMA SDK are the original projects. LZMA SDK is roughly
27 a subset of the 7-Zip source tree.
29 p7zip is 7-Zip's command line tools ported to POSIX-like systems.
31 LZMA Utils provide a gzip-like lzma tool for POSIX-like systems.
32 LZMA Utils are based on LZMA SDK. XZ Utils are the successor to
33 LZMA Utils.
35 There are several other projects using LZMA. Most are more or less
36 based on LZMA SDK. See <http://7-zip.org/links.html>.
39 Q: Why is liblzma named liblzma if its primary file format is .xz?
40 Shouldn't it be e.g. libxz?
42 A: When the designing of the .xz format began, the idea was to replace
43 the .lzma format and use the same .lzma suffix. It would have been
44 quite OK to reuse the suffix when there were very few .lzma files
45 around. However, the old .lzma format become popular before the
46 new format was finished. The new format was renamed to .xz but the
47 name of liblzma wasn't changed.
50 Q: Do XZ Utils support the .7z format?
52 A: No. Use 7-Zip (Windows) or p7zip (POSIX-like systems) to handle .7z
53 files.
56 Q: I have many .tar.7z files. Can I convert them to .tar.xz without
57 spending hours recompressing the data?
59 A: In the "extra" directory, there is a script named 7z2lzma.bash which
60 is able to convert some .7z files to the .lzma format (not .xz). It
61 needs the 7za (or 7z) command from p7zip. The script may silently
62 produce corrupt output if certain assumptions are not met, so
63 decompress the resulting .lzma file and compare it against the
64 original before deleting the original file!
67 Q: I have many .lzma files. Can I quickly convert them to the .xz format?
69 A: For now, no. Since XZ Utils supports the .lzma format, it's usually
70 not too bad to keep the old files in the old format. If you want to
71 do the conversion anyway, you need to decompress the .lzma files and
72 then recompress to the .xz format.
74 Technically, there is a way to make the conversion relatively fast
75 (roughly twice the time that normal decompression takes). Writing
76 such a tool would take quite a bit time though, and would probably
77 be useful to only a few people. If you really want such a conversion
78 tool, contact Lasse Collin and offer some money.
81 Q: I have installed xz, but my tar doesn't recognize .tar.xz files.
82 How can I extract .tar.xz files?
84 A: xz -dc foo.tar.xz | tar xf -
87 Q: Can I recover parts of a broken .xz file (e.g. corrupted CD-R)?
89 A: It may be possible if the file consists of multiple blocks, which
90 typically is not the case if the file was created in single-threaded
91 mode. There is no recovery program yet.
94 Q: Is (some part of) XZ Utils patented?
96 A: Lasse Collin is not aware of any patents that could affect XZ Utils.
97 However, due to nature of software patents, it's not possible to
98 guarantee that XZ Utils isn't affected by any third party patent(s).
101 Q: Where can I find documentation about the file format and algorithms?
103 A: The .xz format is documented in xz-file-format.txt. It is a container
104 format only, and doesn't include descriptions of any non-trivial
105 filters.
107 Documenting LZMA and LZMA2 is planned, but for now, there is no other
108 documentation that the source code. Before you begin, you should know
109 the basics of LZ77 and range coding algorithms. LZMA is based on LZ77,
110 but LZMA is a lot more complex. Range coding is used to compress
111 the final bitstream like Huffman coding is used in Deflate.
114 Q: I cannot find BCJ and BCJ2 filters. Don't they exist in liblzma?
116 A: BCJ filter is called "x86" in liblzma. BCJ2 is not included,
117 because it requires using more than one encoded output stream.
118 A streamable version of BCJ2-style filtering is planned.
121 Q: I need to use a script that runs "xz -9". On a system with 256 MiB
122 of RAM, xz says that it cannot allocate memory. Can I make the
123 script work without modifying it?
125 A: Set a default memory usage limit for compression. You can do it e.g.
126 in a shell initialization script such as ~/.bashrc or /etc/profile:
128 XZ_DEFAULTS=--memlimit-compress=150MiB
129 export XZ_DEFAULTS
131 xz will then scale the compression settings down so that the given
132 memory usage limit is not reached. This way xz shouldn't run out
133 of memory.
135 Check also that memory-related resource limits are high enough.
136 On most systems, "ulimit -a" will show the current resource limits.
139 Q: How do I create files that can be decompressed with XZ Embedded?
141 A: See the documentation in XZ Embedded. In short, something like
142 this is a good start:
144 xz --check=crc32 --lzma2=preset=6e,dict=64KiB
146 Or if a BCJ filter is needed too, e.g. if compressing
147 a kernel image for PowerPC:
149 xz --check=crc32 --powerpc --lzma2=preset=6e,dict=64KiB
151 Adjust dictionary size to get a good compromise between
152 compression ratio and decompressor memory usage. Note that
153 in single-call decompression mode of XZ Embedded, a big
154 dictionary doesn't increase memory usage.
157 Q: Will xz support threaded compression?
159 A: It is planned and has been taken into account when designing
160 the .xz file format. Eventually there will probably be three types
161 of threading, each method having its own advantages and disadvantages.
163 The simplest method is splitting the uncompressed data into blocks
164 and compressing them in parallel independent from each other.
165 Since the blocks are compressed independently, they can also be
166 decompressed independently. Together with the index feature in .xz,
167 this allows using threads to create .xz files for random-access
168 reading. This also makes threaded decompression possible, although
169 it is not clear if threaded decompression will ever be implemented.
171 The independent blocks method has a couple of disadvantages too. It
172 will compress worse than a single-block method. Often the difference
173 is not too big (maybe 1-2 %) but sometimes it can be too big. Also,
174 the memory usage of the compressor increases linearly when adding
175 threads.
177 Match finder parallelization is another threading method. It has
178 been in 7-Zip for ages. It doesn't affect compression ratio or
179 memory usage significantly. Among the three threading methods, only
180 this is useful when compressing small files (files that are not
181 significantly bigger than the dictionary). Unfortunately this method
182 scales only to about two CPU cores.
184 The third method is pigz-style threading (I use that name, because
185 pigz <http://www.zlib.net/pigz/> uses that method). It doesn't
186 affect compression ratio significantly and scales to many cores.
187 The memory usage scales linearly when threads are added. It isn't
188 significant with pigz, because Deflate uses only 32 KiB dictionary,
189 but with LZMA2 the memory usage will increase dramatically just like
190 with the independent blocks method. There is also a constant
191 computational overhead, which may make pigz-method a bit dull on
192 dual-core compared to the parallel match finder method, but with more
193 cores the overhead is not a big deal anymore.
195 Combining the threading methods will be possible and also useful.
196 E.g. combining match finder parallelization with pigz-style threading
197 can cut the memory usage by 50 %.
199 It is possible that the single-threaded method will be modified to
200 create files indentical to the pigz-style method. We'll see once
201 pigz-style threading has been implemented in liblzma.
204 Q: How do I build a program that needs liblzmadec (lzmadec.h)?
206 A: liblzmadec is part of LZMA Utils. XZ Utils has liblzma, but no
207 liblzmadec. The code using liblzmadec should be ported to use
208 liblzma instead. If you cannot or don't want to do that, download
209 LZMA Utils from <http://tukaani.org/lzma/>.
212 Q: The default build of liblzma is too big. How can I make it smaller?
214 A: Give --enable-small to the configure script. Use also appropriate
215 --enable or --disable options to include only those filter encoders
216 and decoders and integrity checks that you actually need. Use
217 CFLAGS=-Os (with GCC) or equivalent to tell your compiler to optimize
218 for size. See INSTALL for information about configure options.
220 If the result is still too big, take a look at XZ Embedded. It is
221 a separate project, which provides a limited but significantly
222 smaller XZ decoder implementation than XZ Utils. You can find it
223 at <http://tukaani.org/xz/embedded.html>.