Stephen Harris wrote: > "Almost always" is very dependent on the disks and size of the array. > > Let's take a 20TiByte array as an example. > ... he did say 'very large'. note, raid10 has another parameter... say you have a 20 drive raid10 of 1TB drives (10TB total usable). if one drive fails, a rebuild only requires reading one drive and writing the hotspare replacement, this is fairly quick compared with the massive restripe operation of a raid5. and, if during that rebuild operation, another drive fails, there's only a 1 in 19 odds of it being the mirror of the previously failed drive if we assume failures are a totally random occurance (yeah, ok, if we assume that a drive is more likely to fail when its being accessed, then the odds are soemwhat higher tha mirror would fail then another drive in the array.... but, an array that does periodic sweeps on idle storage will greatly reduce the possibility of this by 'discovering' a failing drive much sooner.