Re: RAID 0+1 OR 1+0 ?

BELLANI, Gabriela wrote: > > Hi, > What are the differences, advantages or disadvantages of: > > - Mirroring first and then stripping > over > - Generating stripe sets and then mirroring > > I'm movimg my database from a SPARC Storage Array of 30 x 2.1 GB to one > of 14 x 9.1 GB > I used to stripe over 3 disks and then mirror the stripe, but I have > less disks to play with in the new configuration. Any experiences to > share ? Here is the difference: RAID 0+1 (ie mirroring two stripe sets to each other): If a drive fails one side of the mirror, an entire stripe set is useless and the whole stripe needs to be recovered from mirror. Also you can only recover from multiple drive failures if all failed drives are on the same side of the mirror. (Probability of data loss from multiple drive failure is 50%.) This can be a real problem if you buy drives in large orders which can come from the same manufacturing batch and may all be prone to the same failure. (Don't poo poo, it happened to us! A whole batch of IBM 2GB drives all failed at 100,000 hours of operation we had as many as 20 drives failing on the same day until they were all replaced. We got lucky and data loss was minimal.) The performance penalty during drive rebuild can be as much as 90% of capacity. Also until the drive is replaced the read performance boost that mirroring gives you is completely lost. RAID 1+0 (ie stripe several mirrored pairs together): If a drive fails only that one drive needs to be rebuild and the read performance boost that mirroring gives you is only lost for that one pair - until the drive is replaced other pairs continue to perform. During drive replacement the performance penalty cannot exceed about 90% of 1/N% of capacity where N is the number of pairs in the stripe set. In addition you are protected against multiple drive failures even before the defunct drive is replaced unless both drives of a mirrored pair are lost (probability 1/2N). Total rebuild time is 1/Nth of RAID 0+1 also. Art S. Kagel