On Sep 16, 2009, at 4:29 PM, "Marty Scholes" <martyscho...@yahoo.com>
wrote:
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.
There is another type of failure that mirrors help with and that is
controller or path failures. If one side of a mirror set is on one
controller or path and the other on another then a failure of one will
not take down the set.
You can't get that with RAIDZn.
-Ross
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