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Re: Big Numbers


SteveJ Aug 28, 2012 11:44 PM
Posted in group: Guerrilla Capacity Planning
Darryl,

Thanks very much for explaining it so patiently to me, A Bear of Little
Brain [reference to Pooh Bear]

cheers
steve

Darryl Gove wrote on 29/08/12 2:53 PM:
> On 28 August 2012 21:05, steve jenkin <stev...@gmail.com> wrote:
>> Darryl Gove wrote on 29/08/12 11:26 AM:
>>
>> The probability of experiencing no failures over 50 years is (99/100)
>> ^ 50. So the probability of experiencing at least one is 1-(99/100)^50
>> = 0.39.
>>
>> Darryl,
>>
>> Thinking a little more on this.
>>
>> If I have a server and run it 4-5 years and there's an averaged change of
>> failure of 6%/year of drives,
>> then over the 5 year life of a single drive:
>>
>>  - Prob. No Failure/yr = 1 - prob(failure/yr) = 1 - .06 = .94
>>  - prob No failure in 5 yr = 0.94 ^ 5 = 0.734
>>
>> Now if I have 3 drives, is the Probability of No drives failing in a single
>> year the product of 1 - sum(prob failure)??
> Not the sum - you can only add the probabilities of mutually exclusive events.
>
>> ie. 0.94 * 0.94 * 0.94 = 0.831
> Yes, this is the probability of no drive failing during one year.
>
>> or 1 - (0.06 + 0.06 +.06) = 1 - 0.18) = .82
> No, this is not right (imagine that the probability of a drive failing is 0.5 :)
>
>> I'm guessing having working out the yearly rate of "No Fails this year" for
>> a group of drives, then the probability for getting through the entire 5
>> years with no failures is the product:
>> ie. p1 * p2 * p3 * p4 * p5
> Yes.
>
> 0.831^5 = 0.396
>
> So the total probability is (prob working for one year)^( #drives * #years )
>
> Darryl.
>
>> cheers
>> steve
>>
>>
>> --
>> Steve Jenkin, Info Tech, Systems and Design Specialist.
>> 0412 786 915 (+61 412 786 915)
>> PO Box 48, Kippax ACT 2615, AUSTRALIA
>>
>> stev...@gmail.com http://members.tip.net.au/~sjenkin
>>
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>


--
Steve Jenkin, Info Tech, Systems and Design Specialist.
0412 786 915 (+61 412 786 915)
PO Box 48, Kippax ACT 2615, AUSTRALIA

stev...@gmail.com http://members.tip.net.au/~sjenkin