From mboxrd@z Thu Jan 1 00:00:00 1970 From: Jeff Moyer Subject: Re: [Lsf-pc] [LSF/MM TOPIC] a few storage topics Date: Mon, 23 Jan 2012 14:19:33 -0500 Message-ID: References: <20120117200609.GA7933@redhat.com> <20120117213648.GA9457@quack.suse.cz> <20120118225808.GA3074@tux1.beaverton.ibm.com> <20120118232200.GA22019@quack.suse.cz> <4F1758D4.9010401@panasas.com> <20120119094637.GA23442@quack.suse.cz> <4F1BFF5F.6000502@panasas.com> <20120123161857.GC28526@quack.suse.cz> <20120123175353.GD30782@redhat.com> <20120123185650.GF30782@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: Jan Kara , Boaz Harrosh , Mike Snitzer , linux-scsi@vger.kernel.org, neilb@suse.de, dm-devel@redhat.com, linux-fsdevel@vger.kernel.org, lsf-pc@lists.linux-foundation.org, "Darrick J. Wong" To: Andrea Arcangeli Return-path: In-Reply-To: <20120123185650.GF30782@redhat.com> (Andrea Arcangeli's message of "Mon, 23 Jan 2012 19:56:50 +0100") Sender: linux-scsi-owner@vger.kernel.org List-Id: linux-fsdevel.vger.kernel.org Andrea Arcangeli writes: > On Mon, Jan 23, 2012 at 01:28:08PM -0500, Jeff Moyer wrote: >> Are you speaking from experience? If so, what workloads were negatively >> affected by merging, and how did you measure that? > > Any workload where two processes compete for accessing the same disk > and one process writes big requests (usually async writes), the other > small (usually sync reads). The one with the small 4k requests > (usually reads) gets some artificial latency if the big requests are > 512k. Vivek did a recent measurement to verify the issue is still > there, and it's basically an hardware issue. Software can't do much > other than possibly reducing the max request size when we notice such > an I/O pattern coming in cfq. I did old measurements that's how I knew > it, but they were so ancient they're worthless by now, this is why > Vivek had to repeat it to verify before we could assume it still > existed on recent hardware. > > These days with cgroups it may be a bit more relevant as max write > bandwidth may be secondary to latency/QoS. Thanks, Vivek was able to point me at the old thread: http://www.spinics.net/lists/linux-fsdevel/msg44191.html Cheers, Jeff