From mboxrd@z Thu Jan 1 00:00:00 1970 From: James Bottomley Subject: Re: [RFC] relaxed barrier semantics Date: Thu, 29 Jul 2010 15:07:17 -0500 Message-ID: <1280434037.4441.414.camel@mulgrave.site> References: <20100728085048.GA8884@lst.de> <4C4FF136.5000205@kernel.org> <20100728090025.GA9252@lst.de> <4C4FF592.9090800@kernel.org> <20100728092859.GA11096@lst.de> <20100729014431.GD4506@thunk.org> <4C51DA1F.2040701@redhat.com> <20100729194904.GA17098@lst.de> <4C51DCF1.3010507@redhat.com> <1280433591.4441.393.camel@mulgrave.site> <20100729200327.GA17767@lst.de> Mime-Version: 1.0 Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: 7bit Cc: Ric Wheeler , Ted Ts'o , Tejun Heo , Vivek Goyal , Jan Kara , jaxboe@fusionio.com, linux-fsdevel@vger.kernel.org, linux-scsi@vger.kernel.org, chris.mason@oracle.com, swhiteho@redhat.com, konishi.ryusuke@lab.ntt.co.jp To: Christoph Hellwig Return-path: Received: from cantor2.suse.de ([195.135.220.15]:45448 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751235Ab0G2UHX (ORCPT ); Thu, 29 Jul 2010 16:07:23 -0400 In-Reply-To: <20100729200327.GA17767@lst.de> Sender: linux-fsdevel-owner@vger.kernel.org List-ID: On Thu, 2010-07-29 at 22:03 +0200, Christoph Hellwig wrote: > On Thu, Jul 29, 2010 at 02:59:51PM -0500, James Bottomley wrote: > > That's basically everything FUA ... you might just as well switch your > > cache to write through and have done. > > > > This, by the way, is one area I'm hoping to have researched on SCSI > > (where most devices do obey the caching directives). Actually see if > > write through without flush barriers is faster than writeback with flush > > barriers. I really suspect it is. > > We have done the research and at least for XFS a write through cache > actually is faster for many workloads. Ric always has workloads where > the cache is faster, though - mostly doing lots of small file write > kind of setups. There's lies, damned lies and benchmarks .. but what I was thinking is could we just do the right thing? SCSI exposes (in sd) the interfaces to change the cache setting, so if the customer *doesn't* specify barriers on mount, could we just flip the device to write through it would be more performant in most use cases. James