Erasure Coding when it lands should be pretty solid.
Until then per-directory data replicas is the killer feature for me (Music has 3, Documents has 5, Downloads has 1). Something to be very excited about with full compression and encryption.
You can do that with ZFS at the cost of defining separate filesystems per directory.
I don't use multiple replicas, but I use that to tailor my backups per directory. ~/documents is snapshotted and backed up on the regular, with long-lived snapshots. Code is snapshotted regularly, but snapshots don't live too long, and they're not shipped to a different drive. I don't care for ~/tmp so no snapshots.
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edit: the cost, besides having to actually create the file systems, is that moving data between them isn't instant.
Until then per-directory data replicas is the killer feature for me (Music has 3, Documents has 5, Downloads has 1). Something to be very excited about with full compression and encryption.