> Yes.  This is a mathematical way of saying
> "lose any P+1 of N disks."

I am hesitant to beat this dead horse, yet it is a nuance that either I have 
completely misunderstood or many people I've met have completely missed.

Whether a stripe of mirrors or mirror of a stripes, any single failure makes 
the array critical, i.e. one failure from disaster.

For example, suppose a stripe of four sets of mirrors.  That stripe has 8 disks 
total: four data and four mirrors.  If one disk fails, say on mirror set 3, 
then set 3 is running on a single disk.  Should that remaining disk in set 3 
fail, the whole stripe is lost.  Yes, the stripe is safe as long as the next 
failure is not from set 3.

Contrast that to RAIDZ3.  Suppose seven total disks with the same effective 
pool size: 4 data and 3 parity.  If any single disk is lost then the array is 
not critical and can still survive any other loss.  In fact, it can survive a 
total of any three disk failures before it becomes critical.

I just see it too often where someone states that a stripe of four mirror sets 
can sustain four disk failures.  Yes, that's true, as long as the correct four 
disks fail.  If we could control which disks fail, then none of this would even 
be necessary, so that argument seems rather silly.
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