150 lines
7.2 KiB
Plaintext
150 lines
7.2 KiB
Plaintext
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History of LZMA Utils and XZ Utils
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==================================
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Tukaani distribution
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In 2005, there was a small group working on Tukaani distribution, which
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was a Slackware fork. One of the project goals was to fit the distro on
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a single 700 MiB ISO-9660 image. Using LZMA instead of gzip helped a
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lot. Roughly speaking, one could fit data that took 1000 MiB in gzipped
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form into 700 MiB with LZMA. Naturally compression ratio varied across
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packages, but this was what we got on average.
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Slackware packages have traditionally had .tgz as the filename suffix,
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which is an abbreviation of .tar.gz. A logical naming for LZMA
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compressed packages was .tlz, being an abbreviation of .tar.lzma.
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At the end of the year 2007, there was no distribution under the
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Tukaani project anymore, but development of LZMA Utils was kept going.
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Still, there were .tlz packages around, because at least Vector Linux
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(a Slackware based distribution) used LZMA for its packages.
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First versions of the modified pkgtools used the LZMA_Alone tool from
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Igor Pavlov's LZMA SDK as is. It was fine, because users wouldn't need
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to interact with LZMA_Alone directly. But people soon wanted to use
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LZMA for other files too, and the interface of LZMA_Alone wasn't
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comfortable for those used to gzip and bzip2.
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First steps of LZMA Utils
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The first version of LZMA Utils (4.22.0) included a shell script called
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lzmash. It was wrapper that had gzip-like command line interface. It
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used the LZMA_Alone tool from LZMA SDK to do all the real work. zgrep,
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zdiff, and related scripts from gzip were adapted work with LZMA and
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were part of the first LZMA Utils release too.
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LZMA Utils 4.22.0 included also lzmadec, which was a small (less than
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10 KiB) decoder-only command line tool. It was written on top of the
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decoder-only C code found from the LZMA SDK. lzmadec was convenient in
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situations where LZMA_Alone (a few hundred KiB) would be too big.
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lzmash and lzmadec were written by Lasse Collin.
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Second generation
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The lzmash script was an ugly and not very secure hack. The last
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version of LZMA Utils to use lzmash was 4.27.1.
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LZMA Utils 4.32.0beta1 introduced a new lzma command line tool written
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by Ville Koskinen. It was written in C++, and used the encoder and
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decoder from C++ LZMA SDK with little modifications. This tool replaced
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both the lzmash script and the LZMA_Alone command line tool in LZMA
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Utils.
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Introducing this new tool caused some temporary incompatibilities,
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because LZMA_Alone executable was simply named lzma like the new
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command line tool, but they had completely different command line
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interface. The file format was still the same.
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Lasse wrote liblzmadec, which was a small decoder-only library based
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on the C code found from LZMA SDK. liblzmadec had API similar to zlib,
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although there were some significant differences, which made it
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non-trivial to use it in some applications designed for zlib and
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libbzip2.
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The lzmadec command line tool was converted to use liblzmadec.
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Alexandre Sauvé helped converting build system to use GNU Autotools.
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This made is easier to test for certain less portable features needed
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by the new command line tool.
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Since the new command line tool never got completely finished (for
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example, it didn't support LZMA_OPT environment variable), the intent
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was to not call 4.32.x stable. Similarly, liblzmadec wasn't polished,
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but appeared to work well enough, so some people started using it too.
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Because the development of the third generation of LZMA Utils was
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delayed considerably (3-4 years), the 4.32.x branch had to be kept
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maintained. It got some bug fixes now and then, and finally it was
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decided to call it stable, although most of the missing features were
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never added.
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File format problems
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The file format used by LZMA_Alone was primitive. It was designed for
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embedded systems in mind, and thus provided only minimal set of
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features. The two biggest problems for non-embedded use were lack of
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magic bytes and integrity check.
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Igor and Lasse started developing a new file format with some help
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from Ville Koskinen. Also Mark Adler, Mikko Pouru, H. Peter Anvin,
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and Lars Wirzenius helped with some minor things at some point of the
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development. Designing the new format took quite a long time (actually,
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too long time would be more appropriate expression). It was mostly
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because Lasse was quite slow at getting things done due to personal
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reasons.
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Originally the new format was supposed to use the same .lzma suffix
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that was already used by the old file format. Switching to the new
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format wouldn't have caused much trouble when the old format wasn't
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used by many people. But since the development of the new format took
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so long time, the old format got quite popular, and it was decided
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that the new file format must use a different suffix.
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It was decided to use .xz as the suffix of the new file format. The
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first stable .xz file format specification was finally released in
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December 2008. In addition to fixing the most obvious problems of
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the old .lzma format, the .xz format added some new features like
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support for multiple filters (compression algorithms), filter chaining
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(like piping on the command line), and limited random-access reading.
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Currently the primary compression algorithm used in .xz is LZMA2.
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It is an extension on top of the original LZMA to fix some practical
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problems: LZMA2 adds support for flushing the encoder, uncompressed
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chunks, eases stateful decoder implementations, and improves support
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for multithreading. Since LZMA2 is better than the original LZMA, the
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original LZMA is not supported in .xz.
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Transition to XZ Utils
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The early versions of XZ Utils were called LZMA Utils. The first
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releases were 4.42.0alphas. They dropped the rest of the C++ LZMA SDK.
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The code was still directly based on LZMA SDK but ported to C and
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converted from callback API to stateful API. Later, Igor Pavlov made
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C version of the LZMA encoder too; these ports from C++ to C were
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independent in LZMA SDK and LZMA Utils.
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The core of the new LZMA Utils was liblzma, a compression library with
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zlib-like API. liblzma supported both the old and new file format. The
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gzip-like lzma command line tool was rewritten to use liblzma.
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The new LZMA Utils code base was renamed to XZ Utils when the name
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of the new file format had been decided. The liblzma compression
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library retained its name though, because changing it would have
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caused unnecessary breakage in applications already using the early
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liblzma snapshots.
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The xz command line tool can emulate the gzip-like lzma tool by
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creating appropriate symlinks (e.g. lzma -> xz). Thus, practically
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all scripts using the lzma tool from LZMA Utils will work as is with
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XZ Utils (and will keep using the old .lzma format). Still, the .lzma
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format is more or less deprecated. XZ Utils will keep supporting it,
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but new applications should use the .xz format, and migrating old
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applications to .xz is often a good idea too.
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