From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1946076AbWGPBox (ORCPT ); Sat, 15 Jul 2006 21:44:53 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1946077AbWGPBox (ORCPT ); Sat, 15 Jul 2006 21:44:53 -0400 Received: from shawidc-mo1.cg.shawcable.net ([24.71.223.10]:3389 "EHLO pd4mo3so.prod.shaw.ca") by vger.kernel.org with ESMTP id S1946076AbWGPBox (ORCPT ); Sat, 15 Jul 2006 21:44:53 -0400 Date: Sat, 15 Jul 2006 19:41:46 -0600 From: Robert Hancock Subject: Re: raid io requests not parallel? In-reply-to: To: Jonathan Baccash Cc: linux-kernel@vger.kernel.org Message-id: <44B9995A.4010402@shaw.ca> MIME-version: 1.0 Content-type: text/plain; charset=ISO-8859-1; format=flowed Content-transfer-encoding: 7bit References: User-Agent: Thunderbird 1.5.0.4 (Windows/20060516) 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. Because it's RAID 1, I'm assuming. The data has to be written to both drives, unlike reads which can be satisfied from either one. -- Robert Hancock Saskatoon, SK, Canada To email, remove "nospam" from hancockr@nospamshaw.ca Home Page: http://www.roberthancock.com/