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 13:28:08 -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> 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: Received: from mx1.redhat.com ([209.132.183.28]:51580 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753539Ab2AWS2q (ORCPT ); Mon, 23 Jan 2012 13:28:46 -0500 In-Reply-To: <20120123175353.GD30782@redhat.com> (Andrea Arcangeli's message of "Mon, 23 Jan 2012 18:53:53 +0100") Sender: linux-fsdevel-owner@vger.kernel.org List-ID: Andrea Arcangeli writes: > On Mon, Jan 23, 2012 at 05:18:57PM +0100, Jan Kara wrote: >> requst granularity. Sure, big requests will take longer to complete but >> maximum request size is relatively low (512k by default) so writing maximum >> sized request isn't that much slower than writing 4k. So it works OK in >> practice. > > Totally unrelated to the writeback, but the merged big 512k requests > actually adds up some measurable I/O scheduler latencies and they in > turn slightly diminish the fairness that cfq could provide with > smaller max request size. Probably even more measurable with SSDs (but > then SSDs are even faster). Are you speaking from experience? If so, what workloads were negatively affected by merging, and how did you measure that? Cheers, Jeff