From mboxrd@z Thu Jan 1 00:00:00 1970 From: Dave Chinner Subject: Re: 12x performance drop on md/linux+sw raid1 due to barriers [xfs] Date: Mon, 15 Dec 2008 10:36:17 +1100 Message-ID: <20081214233617.GD32301@disturbed> References: <493A9BE7.3090001@sandeen.net> <200812131826.25280.Martin@lichtvoll.de> <4943F37B.8080405@sandeen.net> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Return-path: Content-Disposition: inline In-Reply-To: <4943F37B.8080405@sandeen.net> Sender: linux-raid-owner@vger.kernel.org To: Eric Sandeen Cc: Martin Steigerwald , linux-xfs@oss.sgi.com, linux-raid@vger.kernel.org, Alan Piszcz xfs@oss.sgi.com List-Id: linux-raid.ids On Sat, Dec 13, 2008 at 11:40:11AM -0600, Eric Sandeen wrote: > Martin Steigerwald wrote: > > > At the moment it appears to me that disabling write cache may often give > > more performance than using barriers. And this doesn't match my > > expectation of write barriers as a feature that enhances performance. > > Why do you have that expectation? I've never seen barriers advertised > as enhancing performance. :) > > I do wonder why barriers on, write cache off is so slow; I'd have > thought the barriers were a no-op. Maybe I'm missing something. Barriers still enforce ordering in this case, so it affects the elevator algorithm.... Cheers, Dave. -- Dave Chinner david@fromorbit.com