From owner-freebsd-fs@FreeBSD.ORG Thu Jul 26 06:59:28 2007 Return-Path: Delivered-To: freebsd-fs@freebsd.org Received: from mx1.freebsd.org (mx1.freebsd.org [IPv6:2001:4f8:fff6::34]) by hub.freebsd.org (Postfix) with ESMTP id 088E916A41F for ; Thu, 26 Jul 2007 06:59:28 +0000 (UTC) (envelope-from M.S.Powell@salford.ac.uk) Received: from abbe.salford.ac.uk (abbe.salford.ac.uk [146.87.0.10]) by mx1.freebsd.org (Postfix) with SMTP id 7226413C46C for ; Thu, 26 Jul 2007 06:59:27 +0000 (UTC) (envelope-from M.S.Powell@salford.ac.uk) Received: (qmail 12054 invoked by uid 98); 26 Jul 2007 07:59:25 +0100 Received: from 146.87.255.121 by abbe.salford.ac.uk (envelope-from , uid 401) with qmail-scanner-2.01 (clamdscan: 0.90/3775. spamassassin: 3.1.8. Clear:RC:1(146.87.255.121):. Processed in 0.106566 secs); 26 Jul 2007 06:59:25 -0000 Received: from rust.salford.ac.uk (HELO rust.salford.ac.uk) (146.87.255.121) by abbe.salford.ac.uk (qpsmtpd/0.3x.614) with SMTP; Thu, 26 Jul 2007 07:59:25 +0100 Received: (qmail 68238 invoked by uid 1002); 26 Jul 2007 06:59:23 -0000 Received: from localhost (sendmail-bs@127.0.0.1) by localhost with SMTP; 26 Jul 2007 06:59:23 -0000 Date: Thu, 26 Jul 2007 07:59:23 +0100 (BST) From: "Mark Powell" To: Doug Rabson In-Reply-To: <1185389856.3698.11.camel@herring.rabson.org> Message-ID: <20070726075607.W68220@rust.salford.ac.uk> References: <20070725174715.9F47E5B3B@mail.bitblocks.com> <1185389856.3698.11.camel@herring.rabson.org> MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII; format=flowed Cc: freebsd-fs@freebsd.org, Pawel Jakub Dawidek , Mark Powell Subject: Re: ZfS & GEOM with many odd drive sizes X-BeenThere: freebsd-fs@freebsd.org X-Mailman-Version: 2.1.5 Precedence: list List-Id: Filesystems List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Thu, 26 Jul 2007 06:59:28 -0000 On Wed, 25 Jul 2007, Doug Rabson wrote: > On Wed, 2007-07-25 at 10:47 -0700, Bakul Shah wrote: >> Does it really do this? As I understood it, only one of the >> disks in a mirror will be read for a given block. If the >> checksum fails, the same block from the other disk is read >> and checksummed. If all the disks in a mirror are read for >> every block, ZFS read performance would get somewhat worse >> instead of linear scaling up with more disks in a mirror. In >> order to monitor data on both disks one would need to >> periodically run "zpool scrub", no? But that is not >> *continuous* monitoring of the two sides. > > This is of course correct. I should have said "continuously checks the > data which you are actually looking at on a regular basis". The > consistency check is via the block checksum (not comparing the date from > the two sides of the mirror). ACcording to this: http://www.opensolaris.org/jive/thread.jspa?threadID=23093&tstart=0 RAID-Z has to read every drive to be able to checksum a block. Isn't this the reason why RAID-Z random reads are so slow and also the reason the pre-fetcher exists to speed up sequential reads? Cheers. -- Mark Powell - UNIX System Administrator - The University of Salford Information Services Division, Clifford Whitworth Building, Salford University, Manchester, M5 4WT, UK. Tel: +44 161 295 4837 Fax: +44 161 295 5888 www.pgp.com for PGP key