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Crowd-sourcing interpretation of IBM RAID 5 extension paper

Tom Womack

As far as I can see from the paper, it's offering a technique for getting 3N capacity out of five capacity-N discs, with protection against one disc failure in conjunction with two badly-located unreadable sectors.

(whereas RAID6 gives you protection against two whole-disc failures, but if you lose one disc and have unreadable sectors in the same place on two of the others then you've lost that sector)

It seems to involve fifteen reads and five writes per sector write, because it works by looking at groups of sectors on each disc, whilst RAID6 requires three reads (the sector you're overwriting and the two parity sectors) and three writes, so there's a lot more bandwidth used.

Basically this is a paper which has discovered a pretty mathematical pattern, with a dubious justification that it might be relevant for data recovery. It doesn't make sense in a world in which discs tend to fail mechanically rather than to develop individual bad sectors.

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