On Tuesday January 4, ptb@xxxxxxxxxxxxxx wrote: > Peter T. Breuer <ptb@xxxxxxxxxxxxxx> wrote: > > No, call it "p". That is the correct name. And I presume you mean "an > > error", not "a failure". > > I'll do this thoroughly, so you can see how it goes. > > Let > > p = probability of a detectible error occuring on a disk in a unit time > p'= ................ indetectible ..................................... > > Then the probability of an error occuring UNdetected on a n-disk raid > array is > > (n-1)p + np' > > and on a 1 disk system (a 1-disk raid array :) it is > > p' > > OK? (hey, I'm a mathematician, it's obvious to me). It may be obvious, but it is also wrong. But then probability is, I think, the branch of mathematics that has the highest ratio of people who think that understand it to people to actually do (witness the success of lotteries). The probability of an event occurring lies between 0 and 1 inclusive. You have given a formula for a probability which could clearly evaluate to a number greater than 1. So it must be wrong. You have also been very sloppy in your language, or your definitions. What do you mean by a "detectable error occurring"? Is it a bit getting flipped on the media, or the drive detecting a CRC error during read? And what is your senario for an undetectable error happening? My understanding of drive technology and CRCs suggests that undetectable errors don't happen without some sort of very subtle hardware error, or high level software error (i.e. the wrong data was written - and that doesn't really count). NeilBrown - 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
