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Date:      Sun, 19 May 2013 16:05:36 -0400
From:      Paul Kraus <>
Cc:        Ivailo Tanusheff <>, Liste FreeBSD <>
Subject:   Re: ZFS install on a partition
Message-ID:  <>
In-Reply-To: <>
References:  <> <> <> <> <>

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On May 18, 2013, at 10:16 PM, 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 =
> error probabilities manufacturers put into drive datasheets. The =
> of a URE is distinct from and very different from the probability of =
> 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 =3D=3D 8,796,093,022,208 bits

10^15 (in bits) / 1 TB ~=3D 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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