ZFS: 3 devices: what’s the difference between mirror and raidz2

The simple answer is that to mirror something takes almost no processing power - it just writes to the disk a second time. For RAID-Z2, you have to compute an entirely new parity block, which although small CAN bog down the CPU when you have to write large amounts of data quickly.

Mirroring is always the preferred solution for high-speed data, if it's just bulk-storage without fast write speeds, RAID-Z2 is a good alternative that does allow any two drives to die as you allude to.

The other advantage is that mirrored pools can be expanded with more mirrored devices - while a RAID-Z2 can not be expanded - though more RAID-Z2 storage can be added to the pool, it will be two RAID-Z2 storage pools concatenated (in effect) rather than equally split between all the storage and striped.

Don't know about the speed, but here is what I believe running ZFS on RAID would means:

• You lose the benefits of atomic writes because now the RAID controller has the last say on when a write happens to the disk. Which means you rely on the RAID controllers NVRAM.

• ZFS also may get lied to if the data was written improperly. ZFS would have to take the RAID controllers word for it.

• You would also lose repairing files because, from ZFS's point of view, you have a single disk, if the data's integrity is bad, ZFS would have no way to repair it because there is no second copy. (assuming you don't set copies=2)

• If the RAID drives fall out of sync the RAID drive may take some time to sync depending on the journaling ability. ZFS will resilver the data it finds bad and at least some OSs may run a resilver periodically to ensure the integrity. Again because the RAID will only display one drive to ZFS, ZFS can't help with the repair/rebuild.

• You would be able to expand the RAID (if the RAID has the capability) and maybe rebuild the ZFS data across more drives. (For me not a big plus considering the negatives so far.)

• Of course all the snapshot functionality of ZFS would be unaffected (assuming the data doesn't silently get corrupted).

Hardware RAID would almost negate any advantages that ZFS would have. Personally, I wouldn't recommend using anything under ZFS, I would run ZFS on bare metal.

However, if there is an advantage I hadn't considered, I'm open to hearing it.