From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from cuda.sgi.com (cuda3.sgi.com [192.48.176.15]) by oss.sgi.com (8.12.11.20060308/8.12.11/SuSE Linux 0.7) with ESMTP id mBENtVWL031854 for ; Sun, 14 Dec 2008 17:55:31 -0600 Message-ID: <49459CF2.5070400@sandeen.net> Date: Sun, 14 Dec 2008 17:55:30 -0600 From: Eric Sandeen MIME-Version: 1.0 Subject: Re: 12x performance drop on md/linux+sw raid1 due to barriers [xfs] References: <493A9BE7.3090001@sandeen.net> <200812131826.25280.Martin@lichtvoll.de> <4943F37B.8080405@sandeen.net> <20081214233617.GD32301@disturbed> In-Reply-To: <20081214233617.GD32301@disturbed> List-Id: XFS Filesystem from SGI List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Sender: xfs-bounces@oss.sgi.com Errors-To: xfs-bounces@oss.sgi.com To: Eric Sandeen , Martin Steigerwald , linux-xfs@oss.sgi.com, Alan Piszcz xfs@oss.sgi.com Dave Chinner wrote: > 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.... (taking linux-raid off becase at this point it really has nothing to do with the thread). oh, er, so is nobarrier+nowritecache safe or not? If the elevator can reorder for us (even though the drive won't) then a journaling fs which needs these ordering guarantees may still be in trouble? Just when I think I have it all straight... :) (ok, so now nobarrier+nowritecache+noop io scheduler might be an interesting test). -Eric _______________________________________________ xfs mailing list xfs@oss.sgi.com http://oss.sgi.com/mailman/listinfo/xfs