vdev/pool math with combined raidzX vdevs...
Martin Simmons
martin at lispworks.com
Fri Jul 6 16:40:46 UTC 2012
>>>>> On Fri, 6 Jul 2012 03:08:46 -0400, Zaphod Beeblebrox said:
>
> Is there some penalty for not googling some basic stats course? OK.
> This is from memory (hint: you probably should google).
>
> p(f) ... the probably of failure of one drive over some unit time (say
> one year). A two drive RAID-0 array has probability p(2dr0) = 2 *
> p(f) + p(f). That is (for the logic guys): the array fails if either
> drive fails.
This looks wrong to me because it can be > 1 if p(f) > 1/3 :-)
I think it should be
p(2dr0) = (2 * p(f) * (1 - p(f))) + (p(f) * p(f))
I.e. the probability of either drive failing and the other one not failing
plus the probability of both drives failing simultaneously.
Or another way of thinking about it:
p(2dr0) = 1 - ((1 - p(f)) * (1 - p(f)))
I.e. the inverse of the probability of neither drive failing
__Martin
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