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> if 10 of your cloud services each have a reliability of 99.999, all together the reliability is not 99.999. (The answer is 99.99)
by vincentmarle 7y ago
> if 10 of your cloud services each have a reliability of 99.999, all together the reliability is not 99.999.
(The answer is 99.99)
- dragonwriter 7y agoIt's probably not in practice, since that assumes failures are perfectly independent. If they are perfectly correlated, the answer is still 99.999. For most real cases, it will be between those extremes.
- titzer 7y agoI think it depends on how you define availability. 1. Suppose we define availability as "at least one is up". If the failures are completely independent, then the probability of any one being down is 10^-5 (five nines) and the probability of all 10 being down at the same time is (10^-5)^10 = 10^-50 (fifty nines). 2. If we instead define availability as "all 10 are up" (which is essentially equivalent to one failure causes a cascading failure) then in the same scenario where failures are independent, this is (1-10^-5)^10 ~= 99.99% (four nines).