From owner-freebsd-fs@FreeBSD.ORG  Wed Jul 25 18:57:42 2007
Return-Path: <owner-freebsd-fs@FreeBSD.ORG>
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 AB8DF16A41F;
	Wed, 25 Jul 2007 18:57:42 +0000 (UTC) (envelope-from dfr@rabson.org)
Received: from itchy.rabson.org (unknown [IPv6:2001:618:400::50b1:e8f2])
	by mx1.freebsd.org (Postfix) with ESMTP id 2FDDA13C483;
	Wed, 25 Jul 2007 18:57:42 +0000 (UTC) (envelope-from dfr@rabson.org)
Received: from [80.177.232.250] (herring.rabson.org [80.177.232.250])
	by itchy.rabson.org (8.13.3/8.13.3) with ESMTP id l6PIva0C009137;
	Wed, 25 Jul 2007 19:57:38 +0100 (BST) (envelope-from dfr@rabson.org)
From: Doug Rabson <dfr@rabson.org>
To: Bakul Shah <bakul@bitblocks.com>
In-Reply-To: <20070725174715.9F47E5B3B@mail.bitblocks.com>
References: <20070725174715.9F47E5B3B@mail.bitblocks.com>
Content-Type: text/plain
Date: Wed, 25 Jul 2007 19:57:36 +0100
Message-Id: <1185389856.3698.11.camel@herring.rabson.org>
Mime-Version: 1.0
X-Mailer: Evolution 2.10.2 FreeBSD GNOME Team Port 
Content-Transfer-Encoding: 7bit
X-Spam-Status: No, score=-1.4 required=5.0 tests=ALL_TRUSTED autolearn=failed 
	version=3.1.0
X-Spam-Checker-Version: SpamAssassin 3.1.0 (2005-09-13) on itchy.rabson.org
X-Virus-Scanned: ClamAV 0.87.1/3763/Wed Jul 25 16:37:41 2007 on
	itchy.rabson.org
X-Virus-Status: Clean
Cc: freebsd-fs@freebsd.org, Pawel Jakub Dawidek <pjd@freebsd.org>,
	Mark Powell <M.S.Powell@salford.ac.uk>
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 <freebsd-fs.freebsd.org>
List-Unsubscribe: <http://lists.freebsd.org/mailman/listinfo/freebsd-fs>,
	<mailto:freebsd-fs-request@freebsd.org?subject=unsubscribe>
List-Archive: <http://lists.freebsd.org/pipermail/freebsd-fs>
List-Post: <mailto:freebsd-fs@freebsd.org>
List-Help: <mailto:freebsd-fs-request@freebsd.org?subject=help>
List-Subscribe: <http://lists.freebsd.org/mailman/listinfo/freebsd-fs>,
	<mailto:freebsd-fs-request@freebsd.org?subject=subscribe>
X-List-Received-Date: Wed, 25 Jul 2007 18:57:42 -0000

On Wed, 2007-07-25 at 10:47 -0700, Bakul Shah wrote:
> >                 If you do that, ZFS can use its checksums to continually
> > monitor the two sides of your mirrors for consistency and will be able
> > to notice as early as possible when one of the drives goes flakey.
> 
> 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).