On Wed, 16 Jan 2008, Josh Paetzel wrote: > RAID is no substitute for backups. In 12 years of using RAID I've lost > arrays that required multiple drive failures twice. Once was a trio of > PATA drives in a mirror with a hot spare, drive 0 failed, and while the > hot spare was syncing drive 1 failed. The other incident was a RAID 5 > array of SCSI drives with a hot spare.....a bad PSU left scorch marks on > all 4 drives. > > In any case, I realized the importance of backups a long time ago, and > while I can't say I've never lost data I can say it's been a couple > decades since I lost data I cared about. These are examples of non-independence of failures. In my earlier message I said that RAID1 fails catastrophically with probability p² under independence. Under complete dependence it fails with probability p (which is a lot worse - just as bad as using only one drive). I have to hope that RAID1 doesn't show complete dependence in practice because it would be useless if it did. The real probability of catastrophic failure must be somewhere in between p and p². Mike