> There's a bit more to RAID6 then adding a second parity block in a stripe. > Consider the following stripe: > > D1 D2 D3 P1 P2 > > If P1 and P2 represent the same encoding of D1 + D2 + D3 (say, XOR like RAID > 4/5) you cannot withstand the loss of any two drives. If two of the data > drives go down, you are still toast. > > You can withstand two data drives going down by not using the same set of > drives. If P1 = D1+D2 and P2 = D2+D3, you can now withstand any two data > drives going down. But if P1 and D1 go down together, you are still toast. My interpretation was that the cross-encoding of P1=D1+D2 and P2=D2+D3 was a given. Is that wrong? > That said, I'd like to get around to making a RAID6 driver sometime. I > think using small RAID6 chunk sizes and pulling in a full parity page on > each access might get respectable performance. But it will be quite a bit > more hairy than the RAID 5 driver. I'm not sure "respectable performance" would so important with a driver like this. My boss would be very interested in using a RAID6 driver because of the added reliability. We don't care about access speed - the server is on our crummy 10Mbps ethernet, behind a 386 firewall. As long as performance doesn't bottom out to something really abysmal, people willing to make the speed/reliability tradeoff aren't going to be too bothered. Ross Vandegrift ross@willow.seitz.com - 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
