[time-nuts] Meaning of MTBF (was: Reliability of atomic clocks)

Dr. David Kirkby (Kirkby Microwave Ltd) drkirkby at kirkbymicrowave.co.uk
Tue Mar 29 06:57:10 EDT 2016


On 28 March 2016 at 00:32, Attila Kinali <attila at kinali.ch> wrote:

>
> Yes, the MTBF is a very simplicistic measure and there are a couple
> of assumptions in its calculation which do not hold generally (or
> rather, it's rather seldom that they hold).



It get's "interesting" when you look at the MTBF times on hard disks. Some
of the figures quoted in hours related to an MTBF of over 100 years. From
what I read before, this was based on you replacing the drive at the end of
its service life (typically 3 years for consumer drives and 5 years for
enterprise grade disks). So no individual drive was ever expected to last
100 years, but if you kept replacing the drives ever 3~5 years, the average
time of an unexpected failure would be 100 years. I guess its a bit like a
car - the engine might run for 250,000 miles, but if you never change the
oil or the camshaft belt, it is not going to last.

I note Seagate have dropped the use of MTBF:

http://knowledge.seagate.com/articles/en_US/FAQ/174791en?language=en_US

changing to an Annualized Failure Rate  (AFR). I don't think Seagate will
ever get a real measure of this, as in many cases people are just going to
throw a hard disk in the bin if it fails, even if under warranty. In many
cases the warranty is with an OEM, so even if you buy a new drive sold
originally to Dell, you can't return it unless you are Dell. Also with hard
drive capacities growing quite fast, if a drive does fail you will probably
chose to replace it with one of higher capacity.



Dr. David Kirkby Ph.D CEng MIET
Kirkby Microwave Ltd
Registered office: Stokes Hall Lodge, Burnham Rd, Althorne, Essex, CM3 6DT,
UK.
Registered in England and Wales, company number 08914892.
http://www.kirkbymicrowave.co.uk/
Tel: 07910 441670 / +44 7910 441670 (0900 to 2100 GMT only please)


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