mirror of https://git.tukaani.org/xz.git
245 lines
10 KiB
Plaintext
245 lines
10 KiB
Plaintext
|
|
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://xz.tukaani.org/xz-embedded/>.
|
|
|