On Sat, Mar 05, 2022 at 03:09:55PM +0900, Kengo.M wrote: > Hi Folks > > I use mdadm conveniently. > > Let me ask a very very basic question. > > One parity disk can be used in RAID 5 and 2 parity disks can be used in RAID 6. > ZFS RAIDZ-3 (raidz3) can use 3 parity disks. > > Is it difficult to increase the number of parity disks to 4, 5 or more. > If so, is the reason for this because of the time it takes to generate the > parity bits? There was a project, some times ago, with the aim to create a library, initially for btrfs, capable of adding "n" parities. This should have been usable by md as well. The problem is that the Vandermonde matrix might not invertible with more than 2 parities. So, they've to use a Cauchy matrix. It might be that performances are degrading quite a lot. I'm not sure about the status of such library, maybe you can ask in the btrfs mailing list. On the other hand, 3 parities (or more) case might be a bit an overkill. For md could be better to have things like RAID-61 or similar. bye, -- piergiorgio
