[ The PC Guide | Systems and Components Reference Guide | Hard Disk Drives | Hard Disk Performance | Hard Disk Quality and Reliability ]

Mean Time Between Failures (MTBF)

One of the most commonly used statistics related to drive reliability is mean time between failures or MTBF. This value, usually measured in hours, is meant to represent the average amount of time that will pass between random failures on a drive of this type. It is usually in the range of 200,000 to 500,000 hours for modern drives.

This number is very often misinterpreted and misused. Usually, it goes like this: "Gee, a year contains 8,766 hours. That means my 500,000 MTBF drive should last 57 years." Then, amusingly, one of two things happens: either the person actually thinks the drive will last 57 years, or the opposite: they realize this is crazy and so they write off the entire MTBF figure as "obvious exaggeration from the manufacturer and therefore useless". The real answer of course is neither.

It is obviously impossible for any individual hard disk to be tested to anywhere near the amount of time required to provide a MTBF factor near 500,000. MTBF is meant to be an estimated average, based on testing done on many hard disks over a smaller period of time. It should obviously be recognized that these are averages and that they are computed by the manufacturer, but they do have value when used properly.

The MTBF figure is intended to be used in conjunction with the useful service life of the drive, the typical amount of time before the drive enters the period where failures due to component wearout increase. If the MTBF is 500,000 hours and the service life is five years, this means that the drive is supposed to last for five years, and that of a large group of drives operating within this timeframe, on average they will accumulate 500,000 of total run time between failures. Or, you can think of it this way: if you used one of these drives and replaced it every five years with another identical one, in theory it should last 57 years before failing, on average (though I somehow doubt we'll be using 1 to 10 GB spinning-platter hard disk drives in the year 2050. :^) )

Overall, MTBF is a "reasonably interesting" reliability statistic--not something totally useless, but definitely something to be taken with a grain of salt. I personally view the drive's warranty and stated service life to be more indicative of what the manufacturer really thinks of the drive. I personally would rather buy a hard disk with a stated service life of five years and a warranty of three years, than one with a service life of three years and warranty of two years, even if the former has an MTBF of 300,000 hours and the latter one of 500,000 hours.

In the real world, the actual amount of time between failures will depend on many factors, including the operating conditions of the drive and how it is used; this section discusses component life. See this section for more on care of hard drives. Ultimately however, luck is also a factor, so keep those backups current.

Next: Redundant Arrays of Inexpensive Disks (RAID)

Home  -  Search  -  Topics  -  Up