Joe Landman <[email protected]> wrote > Use AFR and warranty, ignore everything else. MTBF does not > correlate > at all against AFR, and AFR is an objective measure.
MTBF is the inverse of the AFR times the number of hours in a year. The specs for a randomly selected Seagate drive are: MTBF hours 1.4 million AFR 0.63% The MTBF is 365 * 24/AFR = 1.39 x 10^6, as they claim. Unfortunately the MTBF is nonsense because the AFR will not stay at 0.63%, and most likely would not be measured at 0.63% at any point during the drive's life by the end users. It is not even clear if the AFR is the average over the 5 year warranty period. It might be the value in the "trough" of the bathtub curve. In any case on a really fantastically reliable set of drives the real MTBF is perhaps 15 years, 10x lower than the spec. Hard to say because the disk spec sheets do not actually disclose where the AFR number came from, and few people keep disks that long. The ratings I would really like the industry to use might be called ef1, ef5, and ef10, where each is the percent of disks that are Expected to be Functioning (defined as: works at full rated speed, has suffered zero data losing events, and still has unused blocks available) at the end of the specified number of years. It would be really easy to compare disks with that system. With AFR etc., not so much. Regards, David Mathog [email protected] Manager, Sequence Analysis Facility, Biology Division, Caltech _______________________________________________ Beowulf mailing list, [email protected] sponsored by Penguin Computing To change your subscription (digest mode or unsubscribe) visit http://www.beowulf.org/mailman/listinfo/beowulf
