ReiserFS is currently supported by 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.
The most-publicised advantage over what was the stock Linux filesystem at the time, Ext2, is that it uses a transaction journal to record changes to filesystem structures. The journal allows the filesystem to quickly return to a consistent state after an unscheduled system shutdown caused by a hard reboot or a system crash. This feature greatly reduces the risk of filesystem corruption (and the need for lengthy filesystem checks). ReiserFS uses a balanced tree (B+ Tree) structure, which allows for storing directories containing huge numbers of small files very efficiently both in terms of speed and disk space used. Compared to Ext2 and Ext3 when dealing with files under 4k, ReiserFS is often faster by a factor of 10–15. This is important, for example, when handling Usenet news spools.
Unfortunately, converting to a ReiserFS filesystem commonly requires users of Ext2 to completely reformat their disks, which is a disadvantage not shared by its main competitor, Ext3. It is more challenging, but there are several utilities that can be used to convert the filesystem without the need to reformat.
Unfortunately, following the arrest and subsequent conviction of Hans Reiser for the murder of his wife, development of the Reiser file systems has languished. As a result, it is no longer the default filesystem for at least some of the distributions mentioned above.
Also See List of Filesystems for more infomation.