Reply to post: From Paper's author Mario Blaum

Crowd-sourcing interpretation of IBM RAID 5 extension paper

Chris Mellor 1

From Paper's author Mario Blaum

Chris,

in your note you write: "The paper, by IBM Almaden researcher Mario Blaum, professes to solve a problem where RAID 5 is insufficient to recover data when two disks fail."

This is an incorrect statement. I never say that in the article. RAID 5 cannot recover data when two disks fail. For that you need RAID 6. The problem addressed in the paper is one disk failure and up to two "silent" failures in sectors. That is, when a disk fails and you start reconstructing using RAID 5, you may find out that you cannot read a sector, and then you have data loss.

In order to handle this situation, people use RAID 6. But this is a very expensive solution, since you need to dedicate a whole second disk to parity. For that reason, we developed the concept of Partial MDS (PMDS) and Sector-Disk (SD) codes, which have redundancy slightly higher than RAID 5 but can handle the situation described.

Let me point out that Microsoft researchers in the Azure system came up independently with a similar solution, though the application was different (our main motivation was arrays of flash memory). Both the IBM and the Microsoft solutions involve computer searches. A theoretical solution was an open problem, which I provide in the paper you mention. The solution involves mathematical concepts like finite fields (to which you refer ironically as a mathematical side line with no real world applicability). I will make no apologies for the math and you are certainly free to believe that this is just a mathematical curiosity. However, we recently presented our results at FAST13 (Plank, Blaum and Hafner) and there was great interest. Jim Plank and I are preparing an expanded version under request for ACM. Best regards.

-Mario Blaum

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