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Comment by barrkel | Hacker News Reader

The storage efficiency gains from RAID5 are not worth the risks, and when you go to RAID6, you lose even more efficiency.

You're better off with RAID10 (only need to read one drive to replace, not all the drives). Better performance all round too.

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I'm not sure I fully understand why RAID10 is better than RAID5. Reading the RAID wikipedia article, they seem to imply that a typical RAID10 setup uses n*2 disks, where each block of data is written to two drives.

But how does that help in the case of drive failure? If a drive fails, then as size increases won't the exposure to a URE also increase? Is it better than RAID5?

Indeed. Reed Solomon coding (maybe only available in system software, not disk hardware) gives you efficiently balanced striping+redundancy

When a drive in a RAID10 array dies, then the data on that drive is still directly available on a drive that's not protected by redundancy, and the other half of the data is still protected. Rebuilding the array requires reading a single drive without error. Rebuilding a RAID5 requires reading n-1 drives and computing parity without error.

In a 4 disk system RAID6 can survive any 2 disks failing but RAID10 can only survive 2 disks failing if you get lucky. Once you go past 4 disks, RAID10 becomes quite expensive and yet can still be killed by a 2 disk failure event.

Work out the probabilities. The results may surprise you. They did me. I was like you once.

The window for losing two disks is reduced though, because rebuilding a single drive failure is much faster in a RAID 10 system where it's just a drive copy, as opposed to n drive reads and a parity calculation.

I work at a hosting company, and we're using RAID10 exclusively.

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