[Beowulf] Are disk MTBF ratings at all useful?

Joe Landman landman at scalableinformatics.com
Thu Apr 18 16:40:50 PDT 2013


On 4/18/2013 7:01 PM, mathog wrote:
> High end SATA and SAS disks claim MTBF values that work out to over 100
> years, and yet it is a common

Amazing isn't it.  Disks that never fail!

> 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!

Statistical analysis (called a bathtub analysis).  MTBF's are WAGs at 
best, and not well matched against empirical observation.

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