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Date:      Sun, 19 May 2013 16:05:36 -0400
From:      Paul Kraus <paul@kraus-haus.org>
To:        kpneal@pobox.com
Cc:        Ivailo Tanusheff <Ivailo.Tanusheff@skrill.com>, Liste FreeBSD <freebsd-questions@freebsd.org>
Subject:   Re: ZFS install on a partition
Message-ID:  <E29A91ED-3B95-402B-B03D-E6C7D319B92B@kraus-haus.org>
In-Reply-To: <20130519021642.GC3841@neutralgood.org>
References:  <F744BBF1-D98C-47BF-9546-14D1A9CB3733@todoo.biz> <372082cab2064846809615a8073e022c@DB3PR07MB059.eurprd07.prod.outlook.com> <A9599DD7-1A32-4607-BC83-2E6E4D03C560@kraus-haus.org> <b179c20ebde742358e2cc52a1f04133e@DB3PR07MB059.eurprd07.prod.outlook.com> <20130519021642.GC3841@neutralgood.org>

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On May 18, 2013, at 10:16 PM, kpneal@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:
>=20
> 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 =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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