This is an interesting idea. You would still have the vulnerability of a stripeset with "A" and "B" where a failure of a single disk would cause the loss of data from two stripesets. Not sure I would want that. If your concerned about utilizing all the drive space on disk drives with different sizes you may be SOL on that unless you use a SAN disk virtualization methodology.

As for figuring out the second parity calculation on RAID 6, what the manufacturers are realizing is that they don't necessarily have to have a different parity algorithm to calculate the second parity. Simply putting the same XOR parity data on two separate disks will provide the same RAID 6 functionality as having a second parity calculation with lower overhead on controllers. The old KISS methodology is coming back into play. I think you will see more and more of the manufacturers going this route.

Phil

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Alternatively, make it two separate 4-drive RAID 5 arrays. To be brutally honest, I tried to read the technical description of how the second parity is calculated, but I can't make sense of it. In our 8-drive scenario, we have a stripe of 6 data blocks, a parity block which is the xor of the 6 data, and some funkified thing I can't wrap my brain around, that somehow allows us to reconstruct any two failed data blocks, so long as we have the other four plus the two parity blocks. And I just don't get how that can work. For the sake of argument, I'll assume it can somehow; it's got to be a hell of a lot more complicated than XOR. I might even recommend the 8 drives be set up as a 7-drive RAID 5 with the 8th drive in the bullpen waiting to be thrown in as a hot spare. When a drive fails, the relief drive would be brought up to speed with the others automagically, and you'd get a text message on your cell phone telling you that a drive had failed in the array. [Tangent alert] Looking at the Drobo (http://www.drobo.com/) has me thinking about how I'd design a software RAID solution to automatically use the space the best way, regardless of how many and how large the drives are.

Let's say we have a cage that can handle up to 8 hot-pluggable drives. A single drive is trivial; it's just a disk. With two or more drives, the system would allocate all the space on the smaller drives to mirror the same amount of space on the larger one in a RAID-1 configuration: D1 AAAAAAA D2 AAAAAAABB D3 AAAAAAABB ... DN AAAAAAABBZZZZZ The areas marked A would be mirrored across all drives. Reads could be done in parallel, fetching different sectors from each drive to get the benefit of striping. The areas marked B would be mirrored across the drives on which they exist, and Z would not be mirrorred at all, so it would not be available except for applications that have specifically asked for the unmirrored filesystem (the box would export several filesystems to the network, each with different redundancy priority; you could use non-redundant space for work files that could be reconstructed if a drive failed.). Now, once all of the A and B areas are nearly full, the system starts converting them, a bit at a time, into RAID 5: D1 aAAAAAA D2 aAAAAAABB D3 aAAAAAABB ... DN aAAAAAABBZZZZZ D1 aaAAAAA D2 aaAAAAABB D3 aaAAAAABB ... DN aaAAAAABBZZZZZ . . . D1 aaaaaaa D2 aaaaaaabb D3 aaaaaaabb ... DN aaaaaaabbZZZZZ At this point, we've got all the drives doing as much RAID-5 as they can, then another chunk of RAID-5 that's missing the smallest drive, etc., until we get to the largest drive that still has that damned unmirrored section. This is as big as it can get without losing redundancy. We might allow the unmirrored area to grow downward and take up some of the space toward the right, since that's the area where parity is the highest percentage of data. But somewhere along the line, we throw another drive into the cage, and the system would automatically mirror the RAID-1 zone, and the RAID-5 gets reconfigured to use the new drive, expanding the space available in the process. To do this right would probably include rewriting LVM code, but it ought to be possible in userspace as a proof of concept.