On Jan 24, 2014, at 11:12 AM, Phil Turmel <philip@xxxxxxxxxx> wrote: > 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". I don't understand this. You're saying it's a "1 URE event in 10^14 bits read" spec? Not a "1 bit nonrecoverable in 10^14 bits read" spec? It seems that a nonrecoverable read error rate of 1 in 2 would mean, 1 bit nonrecoverable per 2 bits read. Same as 512 bits nonrecoverable per 1024 bits read. Same as 1 sector nonrecoverable per 2 sectors read. >> At the moment a Poisson distribution is out of scope because m >> 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. Absolutely. But if I get the much earlier math wrong, then my understanding of the risk will be one or more orders of magnitude wrong. Whether underestimating or overestimating the risk, there are bad consequences. > >>> 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. Be specific, because…. > The specification is a maximum of an average. Stating the average rate is below the max specified rate, is consistent with the spec being a maximum of an average. I don't see where you're getting the average from when there isn't even an X < Y < Z published. All we have is X < Z. > An > average that changes with time, and cannot be measured from single events. On that point we agree. But with identical publish error rate specs we routinely see model drives give us more problems than others, even among the same manufacturer, even sometimes within a model varying by batch. So obviously the spec has a rather massive range to it. Chris Murphy -- 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
