Reiser4

Reiser4 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.As of 2007, 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 much more widely adopted. Reiser4 is also available from Andrew Morton's -mm kernel sources. Linux kernel developers claim that Reiser4 breaks Linux coding standards, but Hans Reiser suggests political reasons.
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