245 lines
10 KiB
Plaintext
245 lines
10 KiB
Plaintext
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XZ Utils FAQ
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============
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Q: What do the letters XZ mean?
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A: Nothing. They are just two letters, which come from the file format
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suffix .xz. The .xz suffix was selected, because it seemed to be
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pretty much unused. It has no deeper meaning.
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Q: What are LZMA and LZMA2?
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A: LZMA stands for Lempel-Ziv-Markov chain-Algorithm. It is the name
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of the compression algorithm designed by Igor Pavlov for 7-Zip.
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LZMA is based on LZ77 and range encoding.
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LZMA2 is an updated version of the original LZMA to fix a couple of
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practical issues. In context of XZ Utils, LZMA is called LZMA1 to
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emphasize that LZMA is not the same thing as LZMA2. LZMA2 is the
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primary compression algorithm in the .xz file format.
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Q: There are many LZMA related projects. How does XZ Utils relate to them?
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A: 7-Zip and LZMA SDK are the original projects. LZMA SDK is roughly
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a subset of the 7-Zip source tree.
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p7zip is 7-Zip's command-line tools ported to POSIX-like systems.
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LZMA Utils provide a gzip-like lzma tool for POSIX-like systems.
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LZMA Utils are based on LZMA SDK. XZ Utils are the successor to
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LZMA Utils.
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There are several other projects using LZMA. Most are more or less
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based on LZMA SDK. See <https://7-zip.org/links.html>.
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Q: Why is liblzma named liblzma if its primary file format is .xz?
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Shouldn't it be e.g. libxz?
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A: When the designing of the .xz format began, the idea was to replace
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the .lzma format and use the same .lzma suffix. It would have been
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quite OK to reuse the suffix when there were very few .lzma files
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around. However, the old .lzma format became popular before the
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new format was finished. The new format was renamed to .xz but the
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name of liblzma wasn't changed.
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Q: Do XZ Utils support the .7z format?
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A: No. Use 7-Zip (Windows) or p7zip (POSIX-like systems) to handle .7z
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files.
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Q: I have many .tar.7z files. Can I convert them to .tar.xz without
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spending hours recompressing the data?
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A: In the "extra" directory, there is a script named 7z2lzma.bash which
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is able to convert some .7z files to the .lzma format (not .xz). It
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needs the 7za (or 7z) command from p7zip. The script may silently
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produce corrupt output if certain assumptions are not met, so
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decompress the resulting .lzma file and compare it against the
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original before deleting the original file!
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Q: I have many .lzma files. Can I quickly convert them to the .xz format?
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A: For now, no. Since XZ Utils supports the .lzma format, it's usually
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not too bad to keep the old files in the old format. If you want to
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do the conversion anyway, you need to decompress the .lzma files and
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then recompress to the .xz format.
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Technically, there is a way to make the conversion relatively fast
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(roughly twice the time that normal decompression takes). Writing
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such a tool would take quite a bit of time though, and would probably
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be useful to only a few people. If you really want such a conversion
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tool, contact Lasse Collin and offer some money.
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Q: I have installed xz, but my tar doesn't recognize .tar.xz files.
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How can I extract .tar.xz files?
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A: xz -dc foo.tar.xz | tar xf -
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Q: Can I recover parts of a broken .xz file (e.g. a corrupted CD-R)?
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A: It may be possible if the file consists of multiple blocks, which
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typically is not the case if the file was created in single-threaded
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mode. There is no recovery program yet.
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Q: Is (some part of) XZ Utils patented?
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A: Lasse Collin is not aware of any patents that could affect XZ Utils.
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However, due to the nature of software patents, it's not possible to
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guarantee that XZ Utils isn't affected by any third party patent(s).
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Q: Where can I find documentation about the file format and algorithms?
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A: The .xz format is documented in xz-file-format.txt. It is a container
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format only, and doesn't include descriptions of any non-trivial
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filters.
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Documenting LZMA and LZMA2 is planned, but for now, there is no other
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documentation than the source code. Before you begin, you should know
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the basics of LZ77 and range-coding algorithms. LZMA is based on LZ77,
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but LZMA is a lot more complex. Range coding is used to compress
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the final bitstream like Huffman coding is used in Deflate.
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Q: I cannot find BCJ and BCJ2 filters. Don't they exist in liblzma?
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A: BCJ filter is called "x86" in liblzma. BCJ2 is not included,
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because it requires using more than one encoded output stream.
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Q: I need to use a script that runs "xz -9". On a system with 256 MiB
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of RAM, xz says that it cannot allocate memory. Can I make the
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script work without modifying it?
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A: Set a default memory usage limit for compression. You can do it e.g.
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in a shell initialization script such as ~/.bashrc or /etc/profile:
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XZ_DEFAULTS=--memlimit-compress=150MiB
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export XZ_DEFAULTS
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xz will then scale the compression settings down so that the given
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memory usage limit is not reached. This way xz shouldn't run out
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of memory.
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Check also that memory-related resource limits are high enough.
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On most systems, "ulimit -a" will show the current resource limits.
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Q: How do I create files that can be decompressed with XZ Embedded?
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A: See the documentation in XZ Embedded. In short, something like
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this is a good start:
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xz --check=crc32 --lzma2=preset=6e,dict=64KiB
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Or if a BCJ filter is needed too, e.g. if compressing
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a kernel image for PowerPC:
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xz --check=crc32 --powerpc --lzma2=preset=6e,dict=64KiB
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Adjust the dictionary size to get a good compromise between
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compression ratio and decompressor memory usage. Note that
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in single-call decompression mode of XZ Embedded, a big
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dictionary doesn't increase memory usage.
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Q: How is multi-threaded compression implemented in XZ Utils?
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A: The simplest method is splitting the uncompressed data into blocks
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and compressing them in parallel independent from each other.
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This is currently the only threading method supported in XZ Utils.
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Since the blocks are compressed independently, they can also be
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decompressed independently. Together with the index feature in .xz,
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this allows using threads to create .xz files for random-access
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reading. This also makes threaded decompression possible.
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The independent blocks method has a couple of disadvantages too. It
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will compress worse than a single-block method. Often the difference
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is not too big (maybe 1-2 %) but sometimes it can be too big. Also,
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the memory usage of the compressor increases linearly when adding
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threads.
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At least two other threading methods are possible but these haven't
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been implemented in XZ Utils:
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Match finder parallelization has been in 7-Zip for ages. It doesn't
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affect compression ratio or memory usage significantly. Among the
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three threading methods, only this is useful when compressing small
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files (files that are not significantly bigger than the dictionary).
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Unfortunately this method scales only to about two CPU cores.
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The third method is pigz-style threading (I use that name, because
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pigz <https://www.zlib.net/pigz/> uses that method). It doesn't
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affect compression ratio significantly and scales to many cores.
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The memory usage scales linearly when threads are added. This isn't
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significant with pigz, because Deflate uses only a 32 KiB dictionary,
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but with LZMA2 the memory usage will increase dramatically just like
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with the independent-blocks method. There is also a constant
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computational overhead, which may make pigz-method a bit dull on
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dual-core compared to the parallel match finder method, but with more
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cores the overhead is not a big deal anymore.
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Combining the threading methods will be possible and also useful.
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For example, combining match finder parallelization with pigz-style
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threading or independent-blocks-threading can cut the memory usage
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by 50 %.
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Q: I told xz to use many threads but it is using only one or two
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processor cores. What is wrong?
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A: Since multi-threaded compression is done by splitting the data into
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blocks that are compressed individually, if the input file is too
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small for the block size, then many threads cannot be used. The
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default block size increases when the compression level is
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increased. For example, xz -6 uses 8 MiB LZMA2 dictionary and
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24 MiB blocks, and xz -9 uses 64 MiB LZMA dictionary and 192 MiB
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blocks. If the input file is 100 MiB, xz -6 can use five threads
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of which one will finish quickly as it has only 4 MiB to compress.
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However, for the same file, xz -9 can only use one thread.
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One can adjust block size with --block-size=SIZE but making the
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block size smaller than LZMA2 dictionary is waste of RAM: using
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xz -9 with 6 MiB blocks isn't any better than using xz -6 with
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6 MiB blocks. The default settings use a block size bigger than
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the LZMA2 dictionary size because this was seen as a reasonable
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compromise between RAM usage and compression ratio.
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When decompressing, the ability to use threads depends on how the
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file was created. If it was created in multi-threaded mode then
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it can be decompressed in multi-threaded mode too if there are
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multiple blocks in the file.
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Q: How do I build a program that needs liblzmadec (lzmadec.h)?
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A: liblzmadec is part of LZMA Utils. XZ Utils has liblzma, but no
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liblzmadec. The code using liblzmadec should be ported to use
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liblzma instead. If you cannot or don't want to do that, download
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LZMA Utils from <https://tukaani.org/lzma/>.
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Q: The default build of liblzma is too big. How can I make it smaller?
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A: Give --enable-small to the configure script. Use also appropriate
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--enable or --disable options to include only those filter encoders
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and decoders and integrity checks that you actually need. Use
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CFLAGS=-Os (with GCC) or equivalent to tell your compiler to optimize
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for size. See INSTALL for information about configure options.
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If the result is still too big, take a look at XZ Embedded. It is
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a separate project, which provides a limited but significantly
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smaller XZ decoder implementation than XZ Utils. You can find it
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at <https://tukaani.org/xz/embedded.html>.
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