From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id ; Thu, 1 Mar 2001 11:04:35 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id ; Thu, 1 Mar 2001 11:04:30 -0500 Received: from zeus.kernel.org ([209.10.41.242]:41417 "EHLO zeus.kernel.org") by vger.kernel.org with ESMTP id ; Thu, 1 Mar 2001 11:04:09 -0500 Date: Thu, 1 Mar 2001 16:02:01 +0000 From: "Stephen C. Tweedie" To: Ben LaHaise Cc: "Stephen C. Tweedie" , Martin Rauh , linux-kernel@vger.kernel.org Subject: Re: Writing on raw device with software RAID 0 is slow Message-ID: <20010301160201.P26280@redhat.com> In-Reply-To: <20010301121418.A7647@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline User-Agent: Mutt/1.2i In-Reply-To: ; from bcrl@redhat.com on Thu, Mar 01, 2001 at 10:44:38AM -0500 Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org Hi, On Thu, Mar 01, 2001 at 10:44:38AM -0500, Ben LaHaise wrote: > > On Thu, 1 Mar 2001, Stephen C. Tweedie wrote: > > > Raw IO is always synchronous: it gets flushed to disk before the write > > returns. You don't get any write-behind with raw IO, so the smaller > > the blocksize you write in, the slower things get. > > More importantly, the mainstream raw io code only writes in 64KB chunks > that are unpipelined, which can lead to writes not hitting the drive > before the sector passes under the rw head. You can work around this to > some extent by issuing multiple writes (via threads, or the aio work I've > done) at the expense of atomicity. Also, before we allow locking of > arbitrary larger ios in main memory, we need bean counting to prevent the > obvious DoSes. Yep. There shouldn't be any problem increasing the 64KB size, it's only the lack of accounting for the pinned memory which stopped me increasing it by default. --Stephen