Disk Storage Service Life

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

Another Way to Look at MTBF

MTBF in hours isn't as easy to understand on a gut level as a simple statement of failure rate. Some manufacturers are addressing that by adding the AFR (annualized failure rate) to their specs-the odds of a drive failing over the course of a single year.

From MTBF to AFR

If you can't find the AFR for a given drive, you can easily calculate it from the MTBF. First divide one failure by the MTBF in hours to get failures per hour. To convert that to failures per year, multiply the result by 8,760 hours for an enterprise drive or by 2,400 hours (or whatever number of hours the MTBF is based on) for a desktop drive. To turn the result into a percentage, multiply by 100. For a 1.2 million hour MTBF for an enterprise drive, for example, the APR comes out to 0.73 percent, which means (in theory at least) that you have a 0.73 percent chance of the drive dying in any given year.

MTBF and Service Life

One important point about MTBF is that it holds true only for the service life of the drive. As already mentioned, once a drive reaches the end of its service life, the failure rate goes way up, and the MTBF is no longer meaningful. The MTBF only applies if you keep replacing individual drives at (or before) the end of their service lives-at which point the technology should be much improved, so you'll want to move on to a new drive in any case.


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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