Re: Questions about bitrot and RAID 5/6

[Phil Turmel <philip@turmel.org>]

On 01/24/2014 02:32 PM, Chris Murphy wrote:
>>> So a URE is either 4096 bits nonrecoverable, or 32768 bits 
>>> nonrecoverable, for HDDs. Correct?
>> 
>> Yes.  Note that the specification is for an *event*, not for a
>> specific number of bits lost.  The error rate is not "bits lost per
>> bits read", it is "bits lost event per bits read".
> 
> I don't understand this. You're saying it's a "1 URE event in 10^14
> bits read" spec? Not a "1 bit nonrecoverable in 10^14 bits read"
> spec?
> 
> It seems that a nonrecoverable read error rate of 1 in 2 would mean,
> 1 bit nonrecoverable per 2 bits read. Same as 512 bits nonrecoverable
> per 1024 bits read. Same as 1 sector nonrecoverable per 2 sectors
> read.

I don't know what more to say here.  Your "seems" is not.

[trim /]

>> You are confused.
> 
> Be specific, because….
> 
>> The specification is a maximum of an average.
> 
> Stating the average rate is below the max specified rate, is
> consistent with the spec being a maximum of an average. I don't see
> where you're getting the average from when there isn't even an X < Y
> < Z published. All we have is X < Z.

I think you are also struggling with the fact the rate, on a single
drive, aside from any specification, is *itself* an average.

The manufacturer is stating that that average, which cannot be clearly
understood without grasping how a Poisson distribution works (or similar
distributions), won't exceed a certain value within the warranty life (a
maximum).  To achieve this, the manufacturer will certainly arrange to
keep the average of these averages below the maximum.

>> An average that changes with time, and cannot be measured from
>> single events.
> 
> On that point we agree. But with identical publish error rate specs
> we routinely see model drives give us more problems than others, even
> among the same manufacturer, even sometimes within a model varying by
> batch. So obviously the spec has a rather massive range to it.

To some extent, manufacturers have to make educated guesses about future
performance on new products.  They pay real $ penalties in warranty
claims if they err greatly in one direction, and real $ penalties in
"unnecessary" process equipment if the err greatly in the other direction.

Obviously, some manufacturers have better knowledge of their own
production facilities than others.

Um, I think we're drifting off-topic now.

Phil
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