Re: RAID-6
From: Neil Brown <hidden>
Date: 2002-11-13 22:50:53
On Wednesday November 13, hpa@zytor.com wrote:
DISKS ------------------------------------> 0 4 8 12 PARITY 1 5 9 13 PARITY 2 6 10 14 PARITY 3 7 11 15 PARITY If you only write blocks 0-3 you *have* to read in the 12 data blocks and write out all 4 parity blocks, whereas in RAID-3 you can get away with only writing 5 blocks. [Well, technically you could also do a read-modify-write on the parity, since parity is linear. This would greatly complicate the code.]
We do read-modify-write if it involves fewer pre-reads than reconstruct-write. so in the above scenario, writing blocks 0,1,2,3 would cause a pre-read of those blocks and the 4 parity blocks, and then all 8 blocks would be re-written. NeilBrown