From owner-freebsd-fs@FreeBSD.ORG Sun May 16 01:01:34 2010 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 DB5211065673 for ; Sun, 16 May 2010 01:01:34 +0000 (UTC) (envelope-from jdc@koitsu.dyndns.org) Received: from qmta09.emeryville.ca.mail.comcast.net (qmta09.emeryville.ca.mail.comcast.net [76.96.30.96]) by mx1.freebsd.org (Postfix) with ESMTP id C13D68FC08 for ; Sun, 16 May 2010 01:01:34 +0000 (UTC) Received: from omta23.emeryville.ca.mail.comcast.net ([76.96.30.90]) by qmta09.emeryville.ca.mail.comcast.net with comcast id J0wB1e0021wfjNsA911bqZ; Sun, 16 May 2010 01:01:35 +0000 Received: from koitsu.dyndns.org ([98.248.46.159]) by omta23.emeryville.ca.mail.comcast.net with comcast id J11a1e0073S48mS8j11aMZ; Sun, 16 May 2010 01:01:35 +0000 Received: by icarus.home.lan (Postfix, from userid 1000) id A905B9B419; Sat, 15 May 2010 18:01:33 -0700 (PDT) Date: Sat, 15 May 2010 18:01:33 -0700 From: Jeremy Chadwick To: Bob Friesenhahn Message-ID: <20100516010133.GA52593@icarus.home.lan> References: <4BEF2F9C.7080409@netscape.net> <4BEF3137.4080203@netscape.net> <20100516001351.GA50879@icarus.home.lan> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.20 (2009-06-14) Cc: freebsd-fs@freebsd.org Subject: Re: Quick ZFS mirroring question for non-mirrored pool 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: Sun, 16 May 2010 01:01:34 -0000 On Sat, May 15, 2010 at 07:51:17PM -0500, Bob Friesenhahn wrote: > On Sat, 15 May 2010, Jeremy Chadwick wrote: > >What you have here is the equivalent of RAID-10. It might be more > >helpful to look at the above as a "stripe of mirrors". > > > >In this situation, you might be better off with raidz1 (RAID-5 in > >concept). You should get better actual I/O performance due to ZFS > >distributing the I/O workload across 4 disks rather than 2. At least > >that's how I understand it. > > That would be a reasonable assumption but actual evidence suggests > otherwise. For sequential I/O, mirrors and raidz1 seem to offer > roughly similar performance, except that mirrors win for reads and > raidz1 often win for writes. The mirror configuration definitely > wins as soon as there are many seeks or multi-user activity. > > The reason why mirrors still do well for sequential I/O is that > there is still load-sharing across the vdevs (smart "striping") but > in full 128K blocks whereas the raidz1 config needs to break the > 128K blocks into smaller blocks which are striped across the disks > in the vdev. Breaking the data into smaller chunks for raidz > multiplies the disk IOPS required. Disk seeks are slow. > > The main reason to choose raidz1 is for better space efficiency but > mirrors offer more performance. > > For an interesting set of results, see the results summary of "Bob's > method" at "http://www.nedharvey.com/". > > The only way to be sure for your own system is to create various > pool configurations and test with something which represents your > expected work load. As long as the pool is not the boot pool, zfs > makes such testing quite easy. Thanks Bob. You're absolutely right. I'd never seen/read said data results before, nor had I read the below material until now; quite interesting and educational. http://blogs.sun.com/roch/entry/when_to_and_not_to -- | Jeremy Chadwick jdc@parodius.com | | Parodius Networking http://www.parodius.com/ | | UNIX Systems Administrator Mountain View, CA, USA | | Making life hard for others since 1977. PGP: 4BD6C0CB |