[Beowulf] Are disk MTBF ratings at all useful?

[mathog]

High end SATA and SAS disks claim MTBF values that work out to over 100 
years, and yet it is a common
observation that certain models fail at rates entirely inconsistent 
with those values.  For instance,
75% of all drives of one model dead in < 6 years.  (Cited by one poster 
in this thread:

https://groups.google.com/forum/#!topic/comp.unix.solaris/zQjoyc8T01Y

).  Additionally, manufacturer warranties at best only go to 5 years, 
which suggests the manufacturers
don't have a whole lot of faith in their MTBF values.

Some of you have huge amounts of storage, how many disk models lasted 
as long as their MTBF suggests
they should?  (Personally we have only one set of disks that are still 
consistent with the claimed MTBF,
a set of 6 Fibre Channel disks that came with a Sun server and are now 
10 years old - with no failures.)

How do they come up with the MTBF values for disks anyway?  Clearly it 
is not based on watching a large
sample of disks for countless years!

Thanks,

David Mathog
mathog at caltech.edu
Manager, Sequence Analysis Facility, Biology Division, Caltech