Date: Tue, 19 Apr 2016 15:30:42 -0500 (CDT) From: "Valeri Galtsev" <galtsev@kicp.uchicago.edu> To: "Steve O'Hara-Smith" <steve@sohara.org> Cc: freebsd-questions@freebsd.org Subject: Re: Raid 1+0 Message-ID: <43635.128.135.52.6.1461097842.squirrel@cosmo.uchicago.edu>
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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@kicp.uchicago.edu> wrote: > >> >> On Tue, April 19, 2016 11:16 am, Lowell Gilbert wrote: >> > "Valeri Galtsev" <galtsev@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@sohara.org> > > _______________________________________________ > freebsd-questions@freebsd.org mailing list > https://lists.freebsd.org/mailman/listinfo/freebsd-questions > To unsubscribe, send any mail to > "freebsd-questions-unsubscribe@freebsd.org" > ++++++++++++++++++++++++++++++++++++++++ Valeri Galtsev Sr System Administrator Department of Astronomy and Astrophysics Kavli Institute for Cosmological Physics University of Chicago Phone: 773-702-4247 ++++++++++++++++++++++++++++++++++++++++ ++++++++++++++++++++++++++++++++++++++++ Valeri Galtsev Sr System Administrator Department of Astronomy and Astrophysics Kavli Institute for Cosmological Physics University of Chicago Phone: 773-702-4247 ++++++++++++++++++++++++++++++++++++++++
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