Reiser FS is a popualar file system developed by Namesys. The term "Reiser" and "ReiserFS" can refer to both versions 3 and 4. This page deals with both.
Version 3 (Known as "ReiserFS")
Take a look at this.
Version 4 (Known as "Reiser4")
ReiserFS is a new "from scratch" version of the ReiserFS file system,
It has the following advanced features:
- efficient support of small files, in terms of disk space and speed
- fast handling of large and very large directories with hundreds of millions of files,
- flexible plugin infrastructure,
- atomic file system modification,
- efficient journaling through wandering logs,
- dynamically optimized disk-layout (through allocate-on-flush, and online repacker)
- Integration of metadata into the file system name-space.
Additionally, Reiser4 introduces dancing trees, a version of B*-trees with the key difference being that underpopulated nodes won't get merged until a flush to disk is forced by low memory or a completed transaction. Such a system also allows Reiser4 to create files and directories without having to waste time and space through fixed blocks. Compare this to Microsoft's FAT filesystem.
Benchmarks performed by Namesys show that Reiser4 is 10 to 15 times faster than ext3 working on files smaller than 1Kb. As of 2004, it is not supported on many Linux distributions, although its predecessor is.