Fossil.

About Fossil

Fossil is the default file system in Plan 9 from Bell Labs. It serves the network protocol 9P and runs as a user space daemon, like most Plan 9 file servers. Fossil is different from most other file systems due to its snapshot/archival feature. It can take snapshots of the entire file system on command or automatically. These snapshots can be kept on the Fossil partition as long as disk space allows; if the partition fills up then old snapshots will be removed to free up disk space. A snapshot can also be saved permanently to Venti. Fossil and Venti are typically installed together.