# Raid 1+0

Valeri Galtsev galtsev at kicp.uchicago.edu
Tue Apr 19 20:30:43 UTC 2016

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On Tue, April 19, 2016 12:07 pm, Steve O'Hara-Smith wrote:
> On Tue, 19 Apr 2016 11:52:20 -0500 (CDT)
> "Valeri Galtsev" <galtsev at kicp.uchicago.edu> wrote:
>
>>
>> On Tue, April 19, 2016 11:16 am, Lowell Gilbert wrote:
>> > "Valeri Galtsev" <galtsev at kicp.uchicago.edu> writes:
>> >
>> >> Somebody with better knowledge of probability theory will correct me
>> if
>> >> I'm wrong some place.
>> >
>> > Well, you are assuming that the probabilities of two drives failing
are entirely independent of each other. The person to whom you are
responding asserted that this is not the case. Neither of you
presented any evidence directly to that point.
>>
>> Correct, we didn't hear proof of one or another. I, however, can not think
>> of any physical mechanism that can be involved which will lead in case of
>> failure of one drive to failure of another. That is why I assume events
are (pretty much) independent.
>
> 	The increased load caused by populating the replacement drive is
> one such physical mechanism. Probability of failure in a period of time
is affected by how busy the drive has been in that period.

As I said, it affects a bit, but not as grossly as you think. Because you
apply the same excessive load every week when you run RAID verification.
Still you don't have all your drives dead even after 3 years (which means
it over 150 verifications).

>
> 	Next Look at the curves for failure probability against age (or
> total activity) and note that the classic bathtub shape and consider
what happens when your whole array is on the steep bit at the end and
the first drive failure happens.
>
> 	Yes random drive failures well within the expected life are pretty
> much independent

Thank you!

Valeri

> but the coupling gets stronger as the drives get older
> due
> to the increasing tendency of load induced failures.
>
> --
> Steve O'Hara-Smith <steve at sohara.org>
>
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Valeri Galtsev
Department of Astronomy and Astrophysics
Kavli Institute for Cosmological Physics
University of Chicago
Phone: 773-702-4247
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Valeri Galtsev