Storage Specs: Disk Drive Failure Rates

By M. David Stone  |  Posted 2008-07-09 Print this article Print


How Likely It Is to Break

The right way to read MTBF is as a statistical statement. It tells you how likely the drive is to fail, or, more precisely, how often a drive will fail, on average. Before we look at the details, though, it's important to understand that not all MTBFs are on equal footing.

Read the Fine Print

Any given MTBF is based on specific testing conditions, including obvious issues like temperature, which can affect the life of the drive. A less obvious but nonetheless critical issue is whether the drive was tested on the assumption that it would run 24 hours a day, 7 days per week (8,760 hours per year), or run just 40 to 50 hours per week (a total of 2,400 hours per year for Seagate Barracuda desktop drives, for example).

The specs for enterprise drives destined for networks are generally based on the first scenario, which also assumes just a few hundred motor starts and stops per year. The specs for drives aimed at the desktop are generally based on the second scenario, and assume thousands of motor starts and stops per year.

Translating MTBF

Depending on the scenario and on how many drives you have, a given MTBF will translate to a different length of time on the calendar (or clock). If a given model drive has a 1.2 million hour MTBF, for example, and you have 1.2 million drives, you can expect an average of one drive to fail every hour. If you have 120 drives-a more reasonable number-you would, on average, expect one to fail every 10,000 hours. That works out to about one every 416 days for enterprise drives running 24 hours per day, or roughly one every 4.2 years for desktop drives running a total of 2,400 hours per year.


M. David Stone is an award-winning freelance writer and computer industry consultant with special areas of expertise in imaging technologies (including printers, monitors, large-screen displays, projectors, scanners, and digital cameras), storage (both magnetic and optical), and word processing. His 25 years of experience in writing about science and technology includes a nearly 20-year concentration on PC hardware and software. He also has a proven track record of making technical issues easy for non-technical readers to understand, while holding the interest of more knowledgeable readers. Writing credits include eight computer-related books, major contributions to four others, and more than 2,000 articles in national and worldwide computer and general interest publications. His two most recent books are The Underground Guide to Color Printers (Addison-Wesley, 1996) and Troubleshooting Your PC, (Microsoft Press, 2000, with co-author Alfred Poor).

Much of David's current writing is for PC Magazine, where he has been a frequent contributor since 1983 and a contributing editor since 1987. His work includes feature articles, special projects, reviews, and both hardware and software solutions for PC Magazine's Solutions columns. He also contributes to other magazines, including Wired. As Computers Editor at Science Digest from 1984 until the magazine stopped publication, he wrote both a monthly column and additional articles. His newspaper column on computers appeared in the Newark Star Ledger from 1995 through 1997.

Non-computer-related work includes the Project Data Book for NASA's Upper Atmosphere Research Satellite (written for GE's Astro-Space Division), and magazine articles and AV productions on subjects ranging from cosmology to ape language experiments. David also develops and writes testing scripts for leading computer magazines, including PC Magazine's PC Labs. His scripts have covered a wide range of subjects, including computers, scanners, printers, modems, word processors, fax modems, and communications software. He lives just outside of New York City, and considers himself a New Yorker at heart.


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