Re: On URE and RAID rebuild - again!

[Gionatan Danti <g.danti@assyoma.it>]

On 05/08/2014 01:29, NeilBrown wrote:
> On Tue, 05 Aug 2014 00:44:04 +0200 Gionatan Danti <g.danti@assyoma.it> 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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