I think I can offer a straightforward explanation to the following: I like the error-correction quality of ZFS; however, the ZFS > Administration Guide states: "A non-redundant pool configuration is > not recommended for production environments even if the single storage > object is presented from a hardware RAID array or from a software > volume manager. ZFS can only detect errors in these configurations." > Does this caveat still apply if the constituent volumes (aka storage > objects) are themselves redundant ZFS pools?
Lets say you have disk targets from two target devices, they could be disk arrays through fibre, SCSI, etc, that present say RAID 5 targets of some sort. If you simply stripe those together with ZFS, it can detect but not fix errors, since it has no ZFS-redundant data to work from. Identical to a single disk running zfs with single copies. However, if you mirrored the RAID 5 targets from the two arrays with ZFS, you would now have ZFS-level redundancy and hence reconstruction capability. If you construct what you propose, the zvols on each of the 3 boxes would have reconstruction ability locally, since that can be done at a block level, but the final, NAS Head zfs volume would have no redundancy or ability to reconstruct data at that level. Richard is right that you would be losing some advantage of ZFS at the high layer, but assuming one of the individual raid-z2's never fail (by losing more than 2 disks) your high level stripe would remain intact.
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