From owner-freebsd-questions@FreeBSD.ORG Sun May 19 20:05:45 2013 Return-Path: Delivered-To: freebsd-questions@freebsd.org Received: from mx1.freebsd.org (mx1.FreeBSD.org [8.8.178.115]) by hub.freebsd.org (Postfix) with ESMTP id A9CE3BF for ; Sun, 19 May 2013 20:05:45 +0000 (UTC) (envelope-from paul@kraus-haus.org) Received: from mail-gh0-f182.google.com (mail-gh0-f182.google.com [209.85.160.182]) by mx1.freebsd.org (Postfix) with ESMTP id 6EEA2809 for ; Sun, 19 May 2013 20:05:44 +0000 (UTC) Received: by mail-gh0-f182.google.com with SMTP id z15so542931ghb.27 for ; Sun, 19 May 2013 13:05:38 -0700 (PDT) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20120113; h=x-received:subject:mime-version:content-type:from:in-reply-to:date :cc:content-transfer-encoding:message-id:references:to:x-mailer :x-gm-message-state; bh=CsTr4fsq3dLrFIJ9/yYFpLARJOf9L63wUmi6GqOb+c4=; b=VdSjxpD2Bf/VpYVtWIruMXN8rA9K0mnpY/xWbl+IgPuT5TWgxTUiNRmm/ATwFA1FVo TfK79mptd1uvE/tC92EKh7ypiqjIZ3UgujVYaZXrJbJ1+kTxT/lO9331l3nOax3t1ljY e7AHmFKdCPBerrFk4uYz6d+HAjEFvLwosBN7fYMRtYb1FkkdLRiEmwO0wyQ0CZmYT9dt /tcX6NfHeyKmPKYJQwc/4+ui9RenAjiG6faUyyoPNdE9zbjDGkhn0NgEztVZ3QFSAmKZ p6ZJcSjni5pfDg5Rb8zNqtsxCvIJzqf6LWq33tK1diYjp+oUoHbNJ9iiKyl7l+1CqYWY 9Pbw== X-Received: by 10.236.138.232 with SMTP id a68mr2300316yhj.29.1368993938166; Sun, 19 May 2013 13:05:38 -0700 (PDT) Received: from [192.168.2.66] ([96.236.21.119]) by mx.google.com with ESMTPSA id v27sm34990533yhj.12.2013.05.19.13.05.36 for (version=TLSv1 cipher=ECDHE-RSA-RC4-SHA bits=128/128); Sun, 19 May 2013 13:05:37 -0700 (PDT) Subject: Re: ZFS install on a partition Mime-Version: 1.0 (Mac OS X Mail 6.3 \(1503\)) Content-Type: text/plain; charset=us-ascii From: Paul Kraus In-Reply-To: <20130519021642.GC3841@neutralgood.org> Date: Sun, 19 May 2013 16:05:36 -0400 Content-Transfer-Encoding: quoted-printable Message-Id: References: <372082cab2064846809615a8073e022c@DB3PR07MB059.eurprd07.prod.outlook.com> <20130519021642.GC3841@neutralgood.org> To: kpneal@pobox.com X-Mailer: Apple Mail (2.1503) X-Gm-Message-State: ALoCoQml14TmEN962KH/KeUGq+0/x+NKM6ACvKHQ6PoEB+T5WP5c9BzSHeSGS/XjOJHZZpMFMz1+ Cc: Ivailo Tanusheff , Liste FreeBSD X-BeenThere: freebsd-questions@freebsd.org X-Mailman-Version: 2.1.14 Precedence: list List-Id: User questions List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Sun, 19 May 2013 20:05:45 -0000 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