On 01/19/2013 05:48 PM, Stan Hoeppner wrote: > On 1/19/2013 1:43 AM, Mikael Abrahamsson wrote: > >> With a BER of 10^-14 you have a 16% risk of getting URE when reading an >> entire 2TB drive. > > On 1/19/2013 7:21 AM, Roy Sigurd Karlsbakk wrote: > >> ok, perhaps, maybe, but then it's 17% chance of losing data after a >> mirror or raid-5 rebuild with 2TB drives... > > > Where are you guys coming up with this 16-17% chance of URE on any > single full read of this 2TB, 10E14 drive? The URE rate here is 1 bit > for every 12.5 trillion bytes. Thus, statistically, one must read this > drive more than 6 times to encounter a URE. Given that, how is any > single full read between the 1st and the 6th going to have a 16-17% > chance of encountering a URE for that one full read? That doesn't make > sense. 2TB/12.5TB == .16 == 16%. It's not quite right, though. A more precise prediction is to use the Poisson distribution[1], as UREs are generally statistically independent of each other (independent of the time since the previous one). For 2TB in a 1:10^14 spec'd drive, it works out to ~ 14.8%. Probability of zero errors in 2TB == P(0, 2TB/12.5TB) == 0.8521. Note that the probability of reading a 12TB array without error given 1:10^14 spec'd drives is P(0, 12TB/12.5TB) ==> 38.29%, not 4%. You can't just scale the error rate by the size of the data to be read. Similarly, the odds of reading through it twice ==> P(0, 24TB/12.5TB) ==> 14.66%. It's not linear. Of course, drives don't have a constant average error rate through their whole life, but they behave as one through most of it. HTH, Phil [1] http://stattrek.com/probability-distributions/poisson.aspx -- To unsubscribe from this list: send the line "unsubscribe linux-raid" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
