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Tilted Cat Head
Administrator
Location: Manhattan, NY
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Mean Time Before Failure...
wonder why they track that statistic on hard drives and other crucial components? Because there are some upper and lower limits to when these parts are going to fail.
Quote:
What is MTBF?
MTBF is the "mean time between failures," that is, the average elapsed time that passes before a failure occurs in a batch of drives under intense test conditions.
The initial inclination of those unfamiliar with the spec is to interpret it as the average expected lifespan of a single drive and either dismiss it as meaningless (bad) or assume this means their favorite drive will last decades (bad).
The SCSI-standard MTBF rating of 1.2 million hours, for example, does not mean that the average unit will run for 137 years before it fails. Likewise, the other extreme would be 1.2 million drives operating for one hour before one failed- equally unlikely.
Rather, MTBF is a product of a large quantity of drives (numbering in the hundreds or low thousands, perhaps) and the number of hours that such a batch runs before experiencing a failure. If a manufacturer places 1000 units to the test and on average manages to squeeze 50 days of operation out of the batch between each individual unit failure, that firm's product has achieved an MTBF of at least 1.2 million hours.
Sometimes, acceleration factors are used for calculating the MTBF of a drive. These are derived from standard statistical methods. Running the test at elevated temperatures beyond "normal," for example, will reduce the time needed to meet a certain number of test hours. Of course, the accuracy of this acceleration factor has a large effect on the final derived MTBF number.
That said, there's obviously some room for maneuver left in a "sweet spot" where firms may attempt to achieve target MTBF by either using more drives or more hours- there's no set unit count or elapsed time that we may regard as standard.
MTBF should be regarded as a minimum statement of reliability by the manufacturer. These days, no manufacturer will spec an enterprise-class drive below 1.2 million hours. Likewise, no firm will bother with MTBF less than 400,000 for a desktop-class disk. It is this consistency, rather than the spec's lack of meaning, that allows one to gloss over these claims.
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