From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-ie0-f176.google.com ([209.85.223.176]:35286 "EHLO mail-ie0-f176.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752304AbaETOHN (ORCPT ); Tue, 20 May 2014 10:07:13 -0400 Received: by mail-ie0-f176.google.com with SMTP id rl12so486535iec.35 for ; Tue, 20 May 2014 07:07:12 -0700 (PDT) Message-ID: <537B6190.2030100@gmail.com> Date: Tue, 20 May 2014 10:07:12 -0400 From: Austin S Hemmelgarn MIME-Version: 1.0 To: russell@coker.com.au, Brendan Hide , linux-btrfs@vger.kernel.org Subject: Re: ditto blocks on ZFS References: <2308735.51F3c4eZQ7@xev> <537A7BF9.5060508@swiftspirit.co.za> <4483661.BdmCOR8JR5@xev> In-Reply-To: <4483661.BdmCOR8JR5@xev> Content-Type: multipart/signed; protocol="application/pkcs7-signature"; micalg=sha1; boundary="------------ms070703080409030108000701" Sender: linux-btrfs-owner@vger.kernel.org List-ID: This is a cryptographically signed message in MIME format. --------------ms070703080409030108000701 Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: quoted-printable On 2014-05-19 22:07, Russell Coker wrote: > On Mon, 19 May 2014 23:47:37 Brendan Hide wrote: >> This is extremely difficult to measure objectively. Subjectively ... s= ee >> below. >> >>> [snip] >>> >>> *What other failure modes* should we guard against? >> >> I know I'd sleep a /little/ better at night knowing that a double disk= >> failure on a "raid5/1/10" configuration might ruin a ton of data along= >> with an obscure set of metadata in some "long" tree paths - but not th= e >> entire filesystem. >=20 > My experience is that most disk failures that don't involve extreme phy= sical=20 > damage (EG dropping a drive on concrete) don't involve totally losing t= he=20 > disk. Much of the discussion about RAID failures concerns entirely fai= led=20 > disks, but I believe that is due to RAID implementations such as Linux = > software RAID that will entirely remove a disk when it gives errors. >=20 > I have a disk which had ~14,000 errors of which ~2000 errors were corre= cted by=20 > duplicate metadata. If two disks with that problem were in a RAID-1 ar= ray=20 > then duplicate metadata would be a significant benefit. >=20 >> The other use-case/failure mode - where you are somehow unlucky enough= >> to have sets of bad sectors/bitrot on multiple disks that simultaneous= ly >> affect the only copies of the tree roots - is an extremely unlikely >> scenario. As unlikely as it may be, the scenario is a very painful >> consequence in spite of VERY little corruption. That is where the >> peace-of-mind/bragging rights come in. >=20 > http://research.cs.wisc.edu/adsl/Publications/corruption-fast08.html >=20 > The NetApp research on latent errors on drives is worth reading. On pa= ge 12=20 > they report latent sector errors on 9.5% of SATA disks per year. So if= you=20 > lose one disk entirely the risk of having errors on a second disk is hi= gher=20 > than you would want for RAID-5. While losing the root of the tree is=20 > unlikely, losing a directory in the middle that has lots of subdirector= ies is=20 > a risk. >=20 > I can understand why people wouldn't want ditto blocks to be mandatory.= But=20 > why are people arguing against them as an option? >=20 >=20 > As an aside, I'd really like to be able to set RAID levels by subtree. = I'd=20 > like to use RAID-1 with ditto blocks for my important data and RAID-0 f= or=20 > unimportant data. >=20 But the proposed changes for n-way replication would already handle this. They would just need the option of having more than one copy per device (which theoretically shouldn't be too hard once you have n-way replication). Also, BTRFS already has the option of replicating the root tree across multiple devices (it is included in the System Data subset), and in fact dose so by default when using multiple devices. 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