On 05/08/2014 01:29, NeilBrown wrote:

On Tue, 05 Aug 2014 00:44:04 +0200 Gionatan Danti [off-list ref] wrote:


Probabilities are often calculated by examining a statistical record - the
two concepts are not separate.
There is probably some theoretical analysis, some statistical analysis, some
marketing and maybe even some actuarial analysis that goes in to the quoted
figure.  I remember when CPU speed was measured in "MIPS".
This stood for
    Meaningless Indicators of Performance for Salesmen

URE rates numbers are probably equally trustworthy.

The probability number doesn't tell you much at all about your drive.
Your drive probably works much better than the quoted rate, but could be much
worse.
The quoted number might say something useful about a collection of 10,000
drives, but if you can afford those, you can probably afford to competent
statistician to explain the details too.



I'm not an electro-magnetic engineer, but I would guess that UREs are caused
by some combination of:
  - irregularities in the physical media
  - imperfections in positioning of the write head
  - fluctuations in temperature and pressure which could
    affect precise performance of resistors and capacitors etc.

and probably various quantum effects that I know nothing about.

Maybe most UREs come from a spec of dust that was in the wrong place at the
wrong time.

If think a better summary would be:
   in normal conditions and typical loads, a collection of 10^14 drives will
   exhibit errors somewhere in the collection on a regular basis.


NeilBrown

VERY informative post. Thank you Neil.

-- 
Danti Gionatan
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Assyoma S.r.l. - www.assyoma.it
email: g.danti@assyoma.it - info@assyoma.it
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