Calculating the Mean Time Between Failures?

Blue Booger asks: "I was looking over some fibrechannel hard drives and noticed that the Mean Time Between Failures was rated at 1.2 million hours. I thought that was pretty high, and figured it up to be close to 137 YEARS!! I went to check some regular IDE drives just for comparison, and they were rated at 500,000 hours (57 years). Now, as I understand it, this is supposed to be the average time that you can expect the drive to last before failures. I rarely have an IDE drive last more than 4 years, and my record is 10 years, so what is the deal? BTW, that is 57 years running 24 hours a day...the MTBF is rated as power on time. Here you can find Western Digital's glossary that defines the term MTBF (pdf). Here you can find a spec sheet on one of their 20GB IDE drives. I checked, and Seagate also lists similar MTBFs. How the heck are they coming up with these numbers?"

Comments

not just drives...

[ryanmoffett]

Cisco used to sell Catalyst 3548XL switches that were listed as having a MTBF of 120,000+ hours. Their current replacement for that line (3550)comes in at 163,000+ hours. We had 7 of 24 3548XL switches fail in the first year we had them. They had poor air flow from a tiny fan, no heatsinks and tons of hot chips. The newer model has the same issue, though they did stuff a cheap foam baffle in the case to get air to flow closer to the chips, none of which have heatsinks. I have no idea how they tested them and got a MTBF of 13 years.

Look at the definition

[aaarrrgggh]

If they run 500 drives for 2,000 hours and observe only one failure, that is a MTBF of 500,000 hours.

Unfortunately, that equation doesn't take into account the fact that some equipment degrades over time; if a product is very reliable for 1,000 hours, and less reliable after that, just double the sample size (maybe triple for statistics), and see what you get.

Real reliability calculations are much more difficult than just what users think MTBF means...

Re:Here's a wild-ass guess

[elmegil]

A lot of hardware vendors actually test before they ship. But aside from that your basic math is about right. A controlled number of units is tested (possibly in stressed environments) and used to build the statistics that say what the expected MTBF should be.

Design Lifetime

[Detritus]

MTBF figures are usually associated with a design lifetime. That hard drive may have a 300,000 hour MTBF based on a 100% duty cycle and a 5 year design lifetime. That tells you the expected failure rate for the first 5 years of operation. After that point, the failure rate may increase rapidly.