From owner-freebsd-questions@FreeBSD.ORG Thu Oct 30 23:12:08 2008 Return-Path: Delivered-To: freebsd-questions@freebsd.org Received: from mx1.freebsd.org (mx1.freebsd.org [IPv6:2001:4f8:fff6::34]) by hub.freebsd.org (Postfix) with ESMTP id E86301065670 for ; Thu, 30 Oct 2008 23:12:08 +0000 (UTC) (envelope-from rich@pencil.math.missouri.edu) Received: from pencil.math.missouri.edu (pencil.math.missouri.edu [128.206.184.195]) by mx1.freebsd.org (Postfix) with ESMTP id B81228FC12 for ; Thu, 30 Oct 2008 23:12:08 +0000 (UTC) (envelope-from rich@pencil.math.missouri.edu) Received: from pencil.math.missouri.edu (localhost [127.0.0.1]) by pencil.math.missouri.edu (8.13.8/8.13.8) with ESMTP id m9UNC80R056395 for ; Thu, 30 Oct 2008 18:12:08 -0500 (CDT) (envelope-from rich@pencil.math.missouri.edu) Received: (from rich@localhost) by pencil.math.missouri.edu (8.13.8/8.13.8/Submit) id m9UNC7Jr056394 for freebsd-questions@freebsd.org; Thu, 30 Oct 2008 18:12:07 -0500 (CDT) (envelope-from rich) Date: Thu, 30 Oct 2008 18:12:07 -0500 From: Rich Winkel To: freebsd-questions@freebsd.org Message-ID: <20081030231207.GA56260@pencil.math.missouri.edu> References: <9f3798c00810291118i1c80cb8cw8d4995eabe6a4f8f@mail.gmail.com> <4908BE2C.7010505@infracaninophile.co.uk> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <4908BE2C.7010505@infracaninophile.co.uk> User-Agent: Mutt/1.4.2.3i Subject: Re: Filesystem, RAID Question X-BeenThere: freebsd-questions@freebsd.org X-Mailman-Version: 2.1.5 Precedence: list List-Id: User questions List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Thu, 30 Oct 2008 23:12:09 -0000 On Wed, Oct 29, 2008 at 07:49:00PM +0000, Matthew Seaman wrote: > Given that you don't have a BBU, what is the status of write caching > on the individual hard drives? You'll have to use 3dm2 or the CLI > equivalent to investigate this, as the RAID controller tends to hide > that level of information from the OS. However, this setting is the > same thing as controlled by the hw.ata.wc sysctl -- and like that > it has a major effect on disk IO performance. Turning write caching > off is the safe, conservative thing to do for maximum data security. Doesn't hw.ata.wc affect only card-level caching? It seems likely that the softupdates queuing order might be scrambled by card-level caching if it juggles pending writes around to minimize seek times. If so, it would be disasterous for data integrity in the event of a power outage. Disk-level caching might be safe though ... Someone needs to ask 3ware whether the card reorders updates and if so, if there's a setting to keep them in order. Rich