On 01/24/2014 12:59 PM, Chris Murphy wrote: > > On Jan 24, 2014, at 10:03 AM, Phil Turmel <philip@xxxxxxxxxx> wrote: >>> w many bits of loss occur with one URE? >> >> Complete physical sector. > > > A complete physical sector represents 512 bytes / 4096 bits, or in > the case of AF disks 4096 bytes / 32768 bits, of loss for one URE. > Correct? > > So a URE is either 4096 bits nonrecoverable, or 32768 bits > nonrecoverable, for HDDs. Correct? Yes. Note that the specification is for an *event*, not for a specific number of bits lost. The error rate is not "bits lost per bits read", it is "bits lost event per bits read". >>>> Your comments suggest you've completely discounted the fact >>>> that published URE rates are now close to, or within, drive >>>> capacities. >>>> >>>> Spend some time with the math and you will be very concerned. >>> >>> Yeah I tried that a year ago and when it came to really super >>> basic questions, no one was willing to answer them and the thread >>> died as if we don't actually know what we're talking about. So I >>> think some rather basic definitions are in order and an agreement >>> that we don't get to redefine mathematics by saying a max error >>> rate is a mean. >>> >>> http://www.spinics.net/lists/raid/msg41669.html >> >> I participated in that thread. Some of your comments there imply >> that the math is simple. It's not (unless you are whiz with >> statistics). Look at the Poisson distribution I referenced and the >> computation examples I gave. > > At the moment a Poisson distribution is out of scope because my > questions have nothing to do with how often, when, or how many, such > URE's will occur. At the moment I only want complete utter clarity on > what a URE/nonrecoverable error (not even the rate) is in terms of > quantity. That's my main problem. Ok, but the earlier arguments in this thread over the relative merits of raid5 versus raid6 very much depend on the error rate. >> Note that a statement about the rate of a randomly occurring error >> is implicitly stating an average. > > Except that it has only one limiter, with the next notch a whole > order magnitude less error. So I don't see how you get an average > unless you're willing to just make assumptions about the bottom end. > It doesn't make sense that a manufacturer would state a maximum error > rate of X and then target that as an average. The average is > certainly well below the max. You are confused. The specification is a maximum of an average. An average that changes with time, and cannot be measured from single events. Phil -- 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
