It has recently come to my attention that most of us in the trade have basically the wrong idea about MTBF. You know the term, right? Mean Time Between Failures - which is a figure we often look at; especially when we choose disks for our arrays. The problem? In the words of the great Inigo Montoya:You keep using that word. I do not think it means what you think it means.
A little reading over at Wikipedia will soon dispell you of the idea that when a disk's spec-sheet says the MTBF is 50 years, that disk will actually last 50 years. Oh, hmm, you say you took a look at that Wikipedia page, and the math scared you? Yeah - me too. So let's ask Richard Elling (Sun.com) to cut through the fog with a real-world example:
MTBF is a summary metric, which hides many important details. For example, data collected for the years 1996-1998 in the US showed that the annual death rate for children aged 5-14 was 20.8 per 100,000 resident population. This shows an average failure rate of 0.0208% per year. Thus, the MTBF for children aged 5-14 in the US is approximately 4,807 years. Clearly, no human child could be expected to live 5,000 years. Similarly, if a vendor says that the disk MTBF is 1 Million hours (114 years), you cannot expect a disk to last that long.
Oh. Well, dang. I guess that disk ain't very likely to last 50 years!
My rule of thumb? Check the warrantee coverage. The manufacturer with the longest warrantee at the lowest price is the one most confident in the durability of their drives.
No comments: