From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S964871AbWGPDWH (ORCPT ); Sat, 15 Jul 2006 23:22:07 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S964873AbWGPDWH (ORCPT ); Sat, 15 Jul 2006 23:22:07 -0400 Received: from liaag1aa.mx.compuserve.com ([149.174.40.27]:7823 "EHLO liaag1aa.mx.compuserve.com") by vger.kernel.org with ESMTP id S964871AbWGPDWG (ORCPT ); Sat, 15 Jul 2006 23:22:06 -0400 Date: Sat, 15 Jul 2006 23:17:35 -0400 From: Chuck Ebbert <76306.1226@compuserve.com> Subject: Re: raid io requests not parallel? To: "Jonathan Baccash" Cc: linux-kernel Message-ID: <200607152320_MC3-1-C51B-11C9@compuserve.com> MIME-Version: 1.0 Content-Transfer-Encoding: 7bit Content-Type: text/plain; charset=us-ascii Content-Disposition: inline Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org In-Reply-To: On Sat, 15 Jul 2006 17:04:57 -0700, Jonathan Baccash wrote: > 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. But it does issue writes in parallel. The problem is in the way RAID1 works. When you do a read from the RAID1, it issues one read request to one of the underlying disks. When you do a write, it issues one write request to _each_ of the underlying disks. So with two disks in the mirror set, every write issued by your program causes two disk writes. -- Chuck "You can't read a newspaper if you can't read." --George W. Bush