Jim Dibb wrote: > I wasn't referring to write corruption but the chance of simultaneous > loss of 2 drives. If you have a drive failure in your R1, it still > needs to be replaced and rebuilt before the second drive also fails. > > 2 drive failure is also what you primarily worry about in R5 systems, > but the chance of two drives failing is larger on R5 because you have > more drives that could possibly fail. > > Once the first drive fails on a R1, the second drive must fail too. > Once the first drive fails on R5, one of N-1 drives must also fail. > If the reliability of any drive is .99 for a given time frame, the > reliability of N of them is (.99)^N. So the second drive in a mirror is > .99 reliable. The set of 3 other drives in a R5 is (.99)^3 or .97.
...and because all the drives are often around the same age and operating in the same environment, the other disks in a R5 array can be "ready to fail" too and the additional load/stress of resyncing after a drive failure can be enough to tip them over the edge. Personally, I have a 6-disk R5 array - 4 + 1 parity + 1 hot spare I'll be going with RAID6 next time - that can withstand the loss of two drives. R. _______________________________________________ Discuss mailing list [email protected] http://lists.slimdevices.com/lists/listinfo/discuss
