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Calculating the Mean Time Between Failures?

Posted by Cliff on from the surely-they-don't-test-for-500k-hours dept.

Blue Booger asks: "I was looking over some fibrechannel hard drives and noticed that the Mean Time Between Failures was rated at 1.2 million hours. I thought that was pretty high, and figured it up to be close to 137 YEARS!! I went to check some regular IDE drives just for comparison, and they were rated at 500,000 hours (57 years). Now, as I understand it, this is supposed to be the average time that you can expect the drive to last before failures. I rarely have an IDE drive last more than 4 years, and my record is 10 years, so what is the deal? BTW, that is 57 years running 24 hours a day...the MTBF is rated as power on time. Here you can find Western Digital's glossary that defines the term MTBF (pdf). Here you can find a spec sheet on one of their 20GB IDE drives. I checked, and Seagate also lists similar MTBFs. How the heck are they coming up with these numbers?"

5 of 100 comments (clear)

  1. Here's a wild-ass guess by HotNeedleOfInquiry · · Score: 4, Insightful
    First they specify a sample period, perhaps a year. Then they multiply the number of units shipped during that time times the estimated hours per year that the drives are run then divide it by the number of units returned due to failure

    For example, shipped 2 million drives last year, each ran 2080 hours ( 8 hours * 52 weeks), roughly 4 trillion hours total. Out of those 2 million units, they got 3466 returns. So the average MTBF was 1.2 million hours.

    --
    "Eve of Destruction", it's not just for old hippies anymore...
  2. Labs by MazTaim · · Score: 4, Insightful

    That's the key word.

    MTBF is probably determined by taking a bunch of drives, putting them into PERFECT conditions that NEVER exist in the real world. Run them in a way that, although test all functionality, really doesn't provide true conditions for drives (IE head always reading/writing up and down the disk probably never seeking, disks always spinning, etc..). Something that drives never do in real life. Statistics...statistics...statistics...(speeling too :)

  3. Marketing BS... by Alomex · · Score: 3, Insightful

    Anybody who has a large number of drives running knows that the figures have become meaningless over time. They use to predict to the T the expected time of failure. They are now a marketing term assuming "a duty cycle" and computed by an absurd "units x time to failure". Using that system, the MTBF of the Honda Civic engine is 100,000 years as there are 1 million Civic's out there and none of them had their engine seize up in the first month.

    Somebody ought to sue them for deceptive advertisement.

  4. No, you are wrong by anthony_dipierro · · Score: 3, Insightful

    Actually, you are wrong... If you have one drive fail per year for 20 years, then the mean time between failures is 10.5 years.

  5. MTBF... by m0rph3us0 · · Score: 3, Insightful

    The best way to determine *REAL* MTBF is how long the drives are warrantied for, no one warranties a product longer than it is supposed to last. When you see a company reduce it's warranty expect quality to drop in accordance.