Examining Disk Storage Reliability Specs

Finding the Service Life

The service life for a drive is typically missing from the spec sheet, but you can often find it in the drive's manual. In most cases, you can search for the manual on the manufacturer's Web site, where it's usually available as a PDF file. You can then search for "service life" in the manual itself.

How Long Is the Warranty?

If you can't find the service life for a particular model drive, and can't get the information from the manufacturer, you might want to simply treat the length of the drive's warranty as the service life. The cynical (some would say, conservative) view is that you should treat it as the service life in any case. After all, the length of the warranty is, by definition, how long the manufacturer is willing to bet the drive will last, regardless of how long it was designed to last.

Back of the Envelope, Please

One thing to keep in mind when comparing MTBFs between drives, even when they are based on the same scenarios, is that they are not solidly reliable numbers based on actual drive history. As a rule, they are based on some limited testing combined with actual results of similar, older models, with the numbers plugged into a mathematical model that calculates the MTBF. Given the same limited data, different mathematical models will spit out different results. It's best to think of the spec as a back-of-the-envelope calculation: a useful indicator, as long as you don't take it too seriously.

Small Differences Don't Matter

The nature of the MTBF spec means that you have to take it with a large grain of salt. A two-to-one difference—600,000 hours versus 1,200,000—is probably meaningful. A 10 or 20 percent difference—800,000 hours versus 1,000,000—may not be.

And in the Real World...

Keep in mind too that even if the MTBF spec were precisely correct, it would only apply in the conditions defined by the testing scenario. As with fuel efficiency claims for cars, your mileage will probably vary. This alone may be enough to explain why some real-world studies have found much higher failure rates than MTBF specs predict. That's an unpleasant reality, but it's important to know.