The ReiserFS is a general-purpose computer file system designed and implemented by a team at Namesys led by Hans Reiser, who is referred to as the project's Benevolent Dictator for Life. It is currently supported for GNU/Linux and may be included in other operating systems in the future. Introduced with version 2.4.1 of the Linux kernel, it was the first journaling file system to be included in the standard kernel. ReiserFS is the default filesystem on the Slackware, SuSE, Xandros, Yoper, Linspire, Kurumin Linux, FTOSX and Libranet Linux distributions.
With the exception of security updates and critical bug fixes, Namesys has ceased development on ReiserFS (now occasionally referred to as Reiser3) to concentrate on its successor, Reiser4.
ReiserFS has fast journaling, which means that you don't spend your life waiting for fsck every time your laptop battery dies, or the UPS for your mission critical server gets its batteries disconnected accidentally by the UPS company's service crew, or your kernel was not as ready for prime time as you hoped, or the silly thing decides you mounted it too many times today.
ReiserFS is based on fast balanced trees. Balanced trees are more robust in their performance, and are a more sophisticated algorithmic foundation for a file system. When we started our project, there was a consensus in the industry that balanced trees were too slow for file system usage patterns. We proved that if you just do them right they are better--take a look at the benchmarks. We have fewer worst case performance scenarios than other file systems and generally better overall performance. If you put 100,000 files in one directory, we think its fine; many other file systems try to tell you that you are wrong to want to do it.
ReiserFS is more space efficient. If you write 100 byte files, we pack many of them into one block. Other file systems put each of them into their own block. We don't have fixed space allocation for inodes. That saves 6% of your disk.
Ok, it's time to fess up. The interesting stuff is still in the future. Because they are nifty, we are going to add database and hypertext like features into the file system. Only by using balanced trees, with their effective handling of small files (database small fields, hypertext keywords), as our technical foundation can we hope to do this. That was our real motivation. As for performance, we may already be slightly better than the traditional file systems (and substantially better than the journaling ones). But they have been tweaking for decades, while we have just got started. This means that over the next few years we are going to improve faster than they are.