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