Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1759773AbYCZQyS (ORCPT ); Wed, 26 Mar 2008 12:54:18 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1757552AbYCZQyA (ORCPT ); Wed, 26 Mar 2008 12:54:00 -0400 Received: from mx1.redhat.com ([66.187.233.31]:49299 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1757368AbYCZQx7 (ORCPT ); Wed, 26 Mar 2008 12:53:59 -0400 Message-ID: <47EA7F1D.9@redhat.com> Date: Wed, 26 Mar 2008 12:51:41 -0400 From: Chris Snook User-Agent: Thunderbird 2.0.0.12 (X11/20080226) MIME-Version: 1.0 To: Emmanuel Florac CC: Bill Davidsen , linux-kernel@vger.kernel.org Subject: Re: RAID-1 performance under 2.4 and 2.6 References: <20080325194306.4ac71ff2@galadriel.home> <47E975F8.3000702@redhat.com> <47E98108.9000906@tmr.com> <47E98712.6090203@redhat.com> <47E98DE4.9000906@tmr.com> <20080326090527.42286e8e@galadriel.home> In-Reply-To: <20080326090527.42286e8e@galadriel.home> Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 8bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1871 Lines: 37 Emmanuel Florac wrote: > Le Tue, 25 Mar 2008 19:42:28 -0400 vous ?criviez: > >> And this is what I was saying earlier, there is a trend to blame the >> benchmark when in fact the same benchmark runs well on 2.4. > > As I mentioned, it looks like 2.4 actually buffers write data on RAID-1 > which is inherently bad (after all if I do RAID-1 it's for the sake of > data integrity, and write caching just counters that). > However, how bad dd may be, it reflects broadly my problem : on small > systems using software RAID, IO is overall way better with 2.4 than > 2.6, especially NFS thruput. > Though I can substantially enhance 2.6 performance through tweaking > (playing with read ahead, disk queue length etc), it still lags behind > 2.4 with defaults settings by a clear margin (10% or more). > This isn't true - fortunately - of larger systems with 12, 24, 48 disks > drives, hardware RAID, Fibre Channel and al. > This sounds more like a VM issue than a RAID issue. I suspect the interesting difference between your small systems and your large systems is the amount of RAM, not the storage. On small systems, the penalty for sizing caches incorrectly is much greater, so small systems tend to suffer more if the default tunings are a little off. If you do some VM tuning (particularly in /proc/sys/vm) and find that it makes a large difference, please do report it. Most of the exciting VM work is targeted to the high end, not the low end, so it's quite possible that the heuristics which choose default VM parameters at boot time are no longer as good for small systems as they once were. -- Chris -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/