I have been looking at versioned filesystems but I can't find any information on how to back them up. If my disk dies and I have to restore from a backup, I want to be able to restore the filesystem completely, including the past versions of all files.
Obviously the traditional methods like cp and rsync won't work, since they'll only copy the current version (right?). The only other thing I can think of would be to unmount the filesystem and use dd, but that's very inconvenient.
Block level backup is your only real hope.
Put your filesystems on top of LVM or a hardware array and then create a snapshot under the filesystem to backup from. Or use dump/dd on the live block device and hope you get something consistent.
FWIW, typical enterprise storage (i.e. NetApp) doesn't normally backup the snapshots of the filesystem - if you're running daily backups, then you already have multiple versions of the file.
What kind of use case are you looking at?