Reiser4

Reiser4, named after creator Hans Reiser, is a computer file system, a new "from scratch" successor to the ReiserFS file system, developed by Namesys and sponsored by DARPA as well as Linspire.

Features

Some of the goals of the Reiser4 file system are:

More efficient journaling through wandering logs
More efficient support of small files, in terms of disk space and speed through block suballocation
Faster handling of directories with large numbers of files
Flexible plugin infrastructure (through which special metadata types, encryption and compression will be supported)
Dynamically optimized disk-layout through allocate-on-flush (also called delayed allocation in XFS)
Transaction support

Some of the more advanced Reiser4 features (such as user-defined transactions) are also not available because of a lack of a VFS API for them.

At present Reiser4 lacks a few standard file system features, such as an online repacker (similar to the defragmentation utilities provided with other file systems). The creators of Reiser4 say they will implement these later; sooner if someone pays them to do so.

Performance

Reiser4 uses B*-trees in conjunction with the dancing tree balancing approach, in which underpopulated nodes will not get merged until a flush to disk except under memory pressure or when a transaction completes. Such a system also allows Reiser4 to create files and directories without having to waste time and space through fixed blocks.

As of 2004, synthetic benchmarks performed by Namesys show that Reiser4 is 10 to 15 times faster than its most serious competitor ext3 working on files smaller than 1 KiB. Namesys's benchmarks suggest it is typically twice the performance of ext3 for general-purpose filesystem usage patterns. Other benchmarks show results of Reiser4 being slower on many operations.

Integration with Linux

As of 2008, Reiser4 has not yet been merged into the mainline Linux kernel and consequently is still not supported on many Linux distributions; however, its predecessor ReiserFS v3 has been widely adopted. Reiser4 is also available from Andrew Morton's -mm kernel sources. Linux kernel developers claim that Reiser4 does not follow Linux coding standards, but Hans Reiser suggests political reasons.

Future of ReiserFS

Hans Reiser was convicted of murder on April 28th, 2008, leaving uncertainty over the future of Reiser4. After his arrest, employees of Namesys assured they would continue to work and that the events would not slow down the software development in the immediate future. In order to afford increasing legal fees, Hans Reiser announced on December 21, 2006 that he was going to sell Namesys; as of March 26, 2008, it has not been sold, although the website is unavailable. In January 2008, Edward Shishkin, an employee of and programmer for Namesys, was quoted in a CNET interview saying that "Commercial activity of Namesys has stopped." Shishkin and others continued the development of Reiser4, making source code available from Shishkin's web site, later relocated to kernel.org.

As of August 2008, Reiser4 development still continues without Namesys. However, kernel developer Theodore Ts'o has suggested btrfs as an alternative for those interested in the design ideas of Reiser4.