[geeks] [JuliaSacks930 at hotmail.com: [janglo] INFO4U: My Ipod Broke My Computer]

[Scott Howard]

On Tue, Jan 17, 2006 at 11:28:54AM -0500, Lionel Peterson wrote:
> > Hard disks have an MTBF of about 1 million hours  (give or take a bit).
> 
> No, more like 300,000 hours, or 34 years of 24x7x365 uptime...

It depends. Basically they range somewhere between about 250k hours and
1.2 million hours (I think there's at least one claiming 1.5 million hours).
1 million seemed a nice round number :)

> But, for every drive that lasts 300,000 hours, doesn't that mean there is 
> one that fails in the first hour, and another drive that lasts 600,000 
> hours?

No.  MTBF is based on a fixed lifetime, which is generally about 5 years.
ie, within the first 5 years of life, the MTBF will be X hours.

What actually makes more sense is the concept of "Expected Annual Failure
Rate" (AFR), which is what most manufacturers actually use themselves as
the metric - but not publically as it doesn't look as impressive :)
As you've said, 300k hours is 34 years, which gives an AFR of about 2.9% -
ie, about 2.9% of all drives will fail each year for the first 5 years.
After that, all bets are off.

For higher quality drives (Most SCSI/FCAL/etc), the MTBF is in the 1.0 to
1.2 million hour mark, giving an AFR of < 1%, or less than 1 out of
every 100 drives failing each year.

AFR is also how most OEM's handle warranty returns. If a drive as an AFR
of 1% and a 3 year warranty, then when you order 100 the manufacturer will
ship you 103 of them. When the OEM has a failure, they are responsible for
the replacement as they have already been shipped the spares. If the OEM
can show a higher fairure rate than the AFR then they can normally go back
to the manufacturer for future replacements (It doesn't happen much, but
it does occasionally).

> Just an question, but can they really warrant the external contacts for that 
> long? (Said another way, do we know that corrosion won't erode the 

Nope, which is why after about 5 years the MTBF means nothing.  That said,
I know people who are running > 10 year old 2Gb SCSI drives.

  Scott.