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List:       linux-raid
Subject:    Re: RAID-6
From:       Neil Brown <neilb () cse ! unsw ! edu ! au>
Date:       2002-11-13 22:50:53
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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
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