On May 18, 2013, at 10:16 PM, kpn...@pobox.com wrote:
> On Sat, May 18, 2013 at 01:29:58PM +0000, Ivailo Tanusheff wrote:
>> Not sure about your calculations, hope you trust them, but in my previous
>> company we have a 3-4 months period when a disk fails almost every day on 2
>> year old servers, so trust me - I do NOT trust those calculations, as I've
>> seen the opposite. Maybe it was a failed batch of disk, shipped in the
>> country, but no one is insured against this. Yes, you can use several hot
>> spares on the software raid, but:
>
> What calculations are you talking about? He posted the uncorrectable read
> error probabilities manufacturers put into drive datasheets. The probability
> of a URE is distinct from and very different from the probability of the
> entire drive failing.
I think he is referring to the calculation I did based on uncorrectable
error rate and whether you will run into that type of error over the life of
the drive.
1 TB == 8,796,093,022,208 bits
10^15 (in bits) / 1 TB ~= 113.687
So if over the life of the drive you READ a TOTAL of 113.687 TB, then
you will, statistically speaking, run into one uncorrectable read error and
potentially return bad data to the application or OS. This does NOT scale with
size of drive, it is the same for all drives with an uncorrectable error rate
of 10^-15 bits. So if you read the entirety of a 1 TB drive 114 times or a 4 TB
29 times you get the same result.
But this is a statistical probability, and some drives will have more
(much more) uncorrectable errors and others will have less (much less),
although I don't know if the distribution falls on a typical gaussian (bell)
curve.
--
Paul Kraus
Deputy Technical Director, LoneStarCon 3
Sound Coordinator, Schenectady Light Opera Company
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