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