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 13:16:27 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id ; Wed, 1 May 2002 13:16:26 -0400 Received: from [212.18.235.99] ([212.18.235.99]:13583 "EHLO street-vision.com") by vger.kernel.org with ESMTP id ; Wed, 1 May 2002 13:16:26 -0400 From: Justin Cormack Message-Id: <200205011716.g41HGNS24162@street-vision.com> Subject: Re: raid1 performance To: kentborg@borg.org (Kent Borg) Date: Wed, 1 May 2002 18:16:22 +0100 (BST) Cc: linux-kernel@vger.kernel.org In-Reply-To: <20020501130127.A10936@borg.org> from "Kent Borg" at May 01, 2002 01:01:27 PM X-Mailer: ELM [version 2.5 PL3] MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org > 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. yes. Look at raid1.c: the code is quite clear. Older versions didnt. > This then brings up two more questions: > > 1. Does the OS even know where the heads are in a modern IDE disk? Not really. But there is probably a vague correspondence. Especially if you havent remapped any bad sectors. > 2. Is "closer" any more finely grained than a binary > positioned/not-positioned? I think so. You can see different performance regions on disks (ie they are faster on the outside for example). You could of course write a program to test seek times from different areas and build up a real locality map. It might not be worth it though. > And I guess another question: How much does RAID 1 help and under what > kinds of usage? the latency is noticeably less in some cases, as the seeks should be smaller on average. I have found this useful sometimes. Justin