I never understood ZFS well.. To me it's like the openstack of file systems. Huge lot of features, nearly too much to be understood by the lambda sysadmin :)

Venti is awesome. It's pifs but actually usable! The thing is that it won't magically backup your filesystem. If I understand correctly, It keeps the data, but you have to keep track of the file indexes yourself (kind of like any filesystem, when you unlink(2), the data remains on disk. Venti just NEVER overwrites it).

I like the idea of using the FS has a way to prevent data loss, but you'll either need to take snapshots very often, or need some kind of version control implemented IN the FS directly.