Mr. James W. Laferriere wrote: > Hello Peter , > > On 11 Nov 2002, H. Peter Anvin wrote: > >>Hi all, >>I'm playing around with RAID-6 algorithms lately. With RAID-6 I mean >>a setup which needs N+2 disks for N disks worth of storage and can >>handle any two disks failing -- this seems to be the contemporary >>definition of RAID-6 (the originally proposed "two-dimensional parity" >>which required N+2*sqrt(N) drives never took off for obvious reasons.) > > Was there a discussion of the 'two-dimensional parity' on the > list ? I don't remember any (of course) . But what other than > 98+2+10 , What was the main difficulty ? I don't (personally) > see any difficulty (other than managability/power/space) to the > ammount of disks required . Tia , JimL > No discussion of two-dimensional parity, but that was the originally proposed RAID-6. Noone ever productized a solution like that to the best of my knowledge. I don't know what you mean with "98+2+10", but the basic problem is that with 2D parity, for N data drives you need 2*sqrt(N) redundancy drives, which for any moderate-sized RAID is a lot (with 9 data drives you need 6 redundancy drives, so you have 67% overhead.) You also have the same kind of performance problems as RAID-4 does, because the "rotating parity" trick of RAID-5 does not work in two dimensions. And for all of this, you're not *guaranteed* more than dual failure recovery (although you might, probabilistically, luck out and have more than that.) P+Q redundancy, the current meaning of RAID-6, instead uses two orthogonal redundancy functions so you only need two redundancy drives regardless of how much data you have, and you can apply the RAID-5 trick of rotating the parity around. So from your 15 drives in the example above, you get 13 drives worth of data instead of 9. -hpa - To unsubscribe from this list: send the line "unsubscribe linux-raid" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
