From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id ; Wed, 1 May 2002 13:01:30 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id ; Wed, 1 May 2002 13:01:29 -0400 Received: from borg.org ([208.218.135.231]:29484 "HELO borg.org") by vger.kernel.org with SMTP id ; Wed, 1 May 2002 13:01:29 -0400 Date: Wed, 1 May 2002 13:01:27 -0400 From: Kent Borg To: =?iso-8859-1?Q?Jakob_=D8stergaard?= , Arjan van de Ven , Jaime Medrano , linux-kernel@vger.kernel.org Subject: Re: raid1 performance Message-ID: <20020501130127.A10936@borg.org> In-Reply-To: <3CCE9038.F4C830B4@redhat.com> <20020430102148.D4470@borg.org> <20020501183553.D31556@unthought.net> Mime-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit User-Agent: Mutt/1.2.5.1i Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org On Wed, May 01, 2002 at 06:35:53PM +0200, Jakob Østergaard wrote: > This is *not* as simple as it sounds. Believe me, I spent a week trying... > > However, with ext2 (and other filesystems as well), a large sequential file > read is *not* sequential on the disk. You should actually see better performance > on RAID-1 than on a single disk for very large reads, becuase some of the lookups > needed (block indirection or whatever) will be run by the "best" disk in the given > situation. Lemme see if I am getting closer. When reading the disk there will be head seeks necessary. When there are two disks, each with its own complete copy of all the data, there is no reason to keep the two disks' heads in the same place. If their heads are in different places, a read can be issued to the disk whose heads are closer to the desired location. This then brings up two more questions: 1. Does the OS even know where the heads are in a modern IDE disk? 2. Is "closer" any more finely grained than a binary positioned/not-positioned? And I guess another question: How much does RAID 1 help and under what kinds of usage? Thanks, -kb, the Kent who is getting smarter.