shapor / tux3 (http://shapor.com/tux3)
Tux3 is a write-anywhere, atomic commit, btree-based versioning filesystem. It is the spiritual and moral successor of Tux2, the most famous filesystem that was never released. The main purpose of Tux3 is to embody Daniel Phillips's new ideas on storage data versioning. The secondary goal is to provide a more efficient snapshotting and replication method for the Zumastor NAS project, and a tertiary goal is to be better than ZFS.
