Why "Double Protecting RAID" (RAID-DP) Doesn't Waste Extra Disk Space
Mike Linett raised a concern in his blog that using NetApp's RAID-DP "wastes storage space" and requires customers to "buy more protection against disk failure".
Actually, using RAID-DP doesn't waste any extra space, and since it's bundled with the base system, there's nothing extra to buy. (For folks who don't follow our technology closely, RAID-DP is a version of RAID that protects against double disk failures by using two parity disks instead of one. Regular RAID only protects against single disk failures. The generic name for double protecting RAIDs is "RAID-6". RAID-DP is our own particular implementation.)
Here's why RAID-DP doesn't waste space. With regular RAID, we recommend that users create 7 disk RAID arrays with one parity disk. With RAID-DP, we recommend that users create 14 disk RAID arrays with 2 parity disks. Either way, it's 2 parity disks for each 14 disk shelf. The math works out that RAID-DP on 14 disks is much, much safer than regular RAID on 7.
The really elegant thing about RAID-DP is that it is backward and forward compatible with our regular RAID. That is, you can take a regular RAID array, add an extra parity drive, and presto you are now double protected. Or if you have a RAID-DP array, and you want the extra parity drive back to increase data capacity, you can downgrade to regular RAID.
Of course, if you convert a 7 disk regular RAID array to RAID-DP, you'll now have an 8 disk array with two parity disks, so your overhead goes from one-out-of-seven to two-out-of-eight, that is from 14% to 25%. To avoid wasting space, you have to create it as a RAID-DP array to start. Either that or convert a 7 disk raid group to RAID-DP and then add 6 more data drives. We thought it would be really cool to be able to take two regular RAID arrays and combine them into a single RAID-DP array, but it was too much work. Oh—and there is essentially no performance penalty. We run all of our benchmarks now with RAID-DP turned on.
Bottom line, I think everyone should use RAID-DP whenever possible.
(For gory technical details on RAID-DP, see here or here. At the conference where they presented their work, the authors Peter Corbett, Bob English, Atul Goel, Tomislav Grcanac, Steven Kleiman, James Leong and Sunitha Sankar won "Best Paper".)
Actually, using RAID-DP doesn't waste any extra space, and since it's bundled with the base system, there's nothing extra to buy. (For folks who don't follow our technology closely, RAID-DP is a version of RAID that protects against double disk failures by using two parity disks instead of one. Regular RAID only protects against single disk failures. The generic name for double protecting RAIDs is "RAID-6". RAID-DP is our own particular implementation.)
Here's why RAID-DP doesn't waste space. With regular RAID, we recommend that users create 7 disk RAID arrays with one parity disk. With RAID-DP, we recommend that users create 14 disk RAID arrays with 2 parity disks. Either way, it's 2 parity disks for each 14 disk shelf. The math works out that RAID-DP on 14 disks is much, much safer than regular RAID on 7.
The really elegant thing about RAID-DP is that it is backward and forward compatible with our regular RAID. That is, you can take a regular RAID array, add an extra parity drive, and presto you are now double protected. Or if you have a RAID-DP array, and you want the extra parity drive back to increase data capacity, you can downgrade to regular RAID.
Of course, if you convert a 7 disk regular RAID array to RAID-DP, you'll now have an 8 disk array with two parity disks, so your overhead goes from one-out-of-seven to two-out-of-eight, that is from 14% to 25%. To avoid wasting space, you have to create it as a RAID-DP array to start. Either that or convert a 7 disk raid group to RAID-DP and then add 6 more data drives. We thought it would be really cool to be able to take two regular RAID arrays and combine them into a single RAID-DP array, but it was too much work. Oh—and there is essentially no performance penalty. We run all of our benchmarks now with RAID-DP turned on.
Bottom line, I think everyone should use RAID-DP whenever possible.
(For gory technical details on RAID-DP, see here or here. At the conference where they presented their work, the authors Peter Corbett, Bob English, Atul Goel, Tomislav Grcanac, Steven Kleiman, James Leong and Sunitha Sankar won "Best Paper".)
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