in particular, the reason why RAID-Z is so much better than mdadm RAID (which is, in turn, IMO much better than most hardware RAID)
Disagree about the hardware raid comment. You say "most hardware RAID", but if you consider the set of hardware RAID implementations that you would actually use on a server, mdadm is pretty feature poor. In particular the advantages of hardware RAID are: . Battery backed write cache. Bcache/flashcache offer this but they have their shortcomings, in particular that most available cache modules are still on top of the SATA channel. . Online resize/reconfigure . BIOS boot support (see recent thread "RAID, again" by me) Linux based BIOS would make some of this better though! If you are saying mdadm is better than "most hardware RAID" where "hardware RAID" is a set of all possible hardware RAID's, including the really crap one's that you wouldn't even consider using on a workstation then I guess I agree, but it's not really a fair comparison ;) Does ZFS have any native support for battery or flash backed write cache? With mdadm I can run bcache on top of it, then lvm on top of that, but with ZFS's tight integration of everything I'm not sure that would be possible and I'd have to run bcache on top of the component disks or the individual zvols, maybe (I'm probably mucking up the zfs terminology here but I hope you know what I mean). James