On 01/24/2014 02:32 PM, Chris Murphy wrote: >>> 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. I don't know what more to say here. Your "seems" is not. [trim /] >> 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. I think you are also struggling with the fact the rate, on a single drive, aside from any specification, is *itself* an average. The manufacturer is stating that that average, which cannot be clearly understood without grasping how a Poisson distribution works (or similar distributions), won't exceed a certain value within the warranty life (a maximum). To achieve this, the manufacturer will certainly arrange to keep the average of these averages below the maximum. >> 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. To some extent, manufacturers have to make educated guesses about future performance on new products. They pay real $ penalties in warranty claims if they err greatly in one direction, and real $ penalties in "unnecessary" process equipment if the err greatly in the other direction. Obviously, some manufacturers have better knowledge of their own production facilities than others. Um, I think we're drifting off-topic now. 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
