From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1750738AbWGPFq7 (ORCPT ); Sun, 16 Jul 2006 01:46:59 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1750739AbWGPFq7 (ORCPT ); Sun, 16 Jul 2006 01:46:59 -0400 Received: from fw5.argo.co.il ([194.90.79.130]:12041 "EHLO argo2k.argo.co.il") by vger.kernel.org with ESMTP id S1750738AbWGPFq7 (ORCPT ); Sun, 16 Jul 2006 01:46:59 -0400 Message-ID: <44B9D2CA.8040306@argo.co.il> Date: Sun, 16 Jul 2006 08:46:50 +0300 From: Avi Kivity User-Agent: Thunderbird 1.5.0.4 (X11/20060614) MIME-Version: 1.0 To: Jonathan Baccash CC: linux-kernel@vger.kernel.org Subject: Re: raid io requests not parallel? References: In-Reply-To: Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit X-OriginalArrivalTime: 16 Jul 2006 05:46:56.0770 (UTC) FILETIME=[3FA34620:01C6A89B] Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org Jonathan Baccash wrote: > > I'm using kernel linux-2.6.15-gentoo-r1, and I noticed performance of > the software RAID-1 is not as good as I would have expected on my two > SATA drives, and I was wondering if anyone has an idea what may be > happening. The test I run is 1024 16k direct-IO reads/writes from > random locations within a 1GB file (on a RAID-1 partition), with my > disk caches set to > write-through mode. In the MT (multi-threaded) case, I issue them from > 8 threads (so it's 128 requests per thread): > > Random read: 10.295 sec > Random write: 19.142 sec > MT Random read: 5.276 sec > MT Random write: 19.839 sec > > As expected, the multi-threaded reads are 2x as fast as single-threaded > reads. But I would have expected (assuming the write to both disks can > occur in parallel) that the random writes are about the same speed (10 > seconds) as the single-threaded random reads, for both the > single-threaded and multi-threaded write cases. The fact that the > multi-threaded reads were > twice as fast indicates to me that read requests can occur in parallel. > > So.... why doesn't the raid issue the writes in parallel? Thanks in > advance for any help. > The writes are issued in parallel, but both disks have to be written. Each head has to service 1024 write requests (compared to just 512 read requests). -- Do not meddle in the internals of kernels, for they are subtle and quick to panic.