From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S264319AbTLQTXC (ORCPT ); Wed, 17 Dec 2003 14:23:02 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S264415AbTLQTXB (ORCPT ); Wed, 17 Dec 2003 14:23:01 -0500 Received: from mail.shareable.org ([81.29.64.88]:49029 "EHLO mail.shareable.org") by vger.kernel.org with ESMTP id S264319AbTLQTW7 (ORCPT ); Wed, 17 Dec 2003 14:22:59 -0500 Date: Wed, 17 Dec 2003 19:22:44 +0000 From: Jamie Lokier To: Linus Torvalds Cc: Helge Hafting , jw schultz , Linux Kernel Mailing List Subject: Re: raid0 slower than devices it is assembled of? Message-ID: <20031217192244.GB12121@mail.shareable.org> References: <200312151434.54886.adasi@kernel.pl> <20031216040156.GJ12726@pegasys.ws> <3FDF1C03.2020509@aitel.hist.no> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.4.1i Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org Linus Torvalds wrote: > My personal guess is that modern RAID0 stripes should be on the order of > several MEGABYTES in size rather than the few hundred kB that most people > use (not to mention the people who have 32kB stripes or smaller - they > just kill their IO access patterns with that, and put the CPU at > ridiculous strain). If a large fs-level I/O transaction is split into lots of 32k transactions by the RAID layer, many of those 32k transactions will be contiguous on the disks. That doesn't mean they're contiguous from the fs point of view, but given that all modern hardware does scatter-gather, shouldn't the contiguous transactions be merged before being sent to the disk? It may strain the CPU (splitting and merging in a different order lots of requests), but I don't see why it should kill I/O access patterns, as they can be as large as if you had large stripes in the first place. -- Jamie