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And so we enter day seven of King's College London major IT outage

Anonymous Coward
Anonymous Coward

> RAID 10 is WORSE than RAID 5. If you lose one disk the remaining disk has to produce every single block, without error, to keep your data alive or to rebuild the RAID. If there is one single URE on the remaining disk your RAID is considered borked and you lose your weekend (at the very least)

RAID10 isn't parity-based like 5 or 6 and thus isn't subject to UREs in the same fashion. Rebuilding a RAID10 stripe just clones block-for-block from one side of the RAID1 to another - that's a remirror rather than a rebuild. Even if there is a block read error reading from one of the drives, a flipped bit in a single block RAID10 isn't the end of the world (and if you've got a checksumming filesystem on top of that it'll be corrected anyway), but with parity-based RAID you've got no way of calculating the new parity from bogus data, so your array is toast.

Remember that, during a parity RAID rebuild, the entire array has to be re-read, parity calculated and re-written to disc - so the bigger your array, the bigger the amount that read and written and the longer rebuild time. RAID10 just needs to clone the contents of one disc to another so no matter the size of your array, it's basically a sequential read of one disk going to a sequential write of another instead of the slower and more random read-modify-write of parity RAIDs.

In a nutshell: as a rule of thumb RAID5|6 rebuild times scale up with the size of the array, RAID10 rebuild times scale with the size of the individual disks.

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