Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S265422AbUFOLgw (ORCPT ); Tue, 15 Jun 2004 07:36:52 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S265426AbUFOLgw (ORCPT ); Tue, 15 Jun 2004 07:36:52 -0400 Received: from ns.virtualhost.dk ([195.184.98.160]:41609 "EHLO virtualhost.dk") by vger.kernel.org with ESMTP id S265422AbUFOLgu (ORCPT ); Tue, 15 Jun 2004 07:36:50 -0400 Date: Tue, 15 Jun 2004 13:36:48 +0200 From: Jens Axboe To: Guillaume =?iso-8859-1?Q?Lac=F4te?= Cc: linux-kernel@vger.kernel.org Subject: Re: BIO ordering and NativeCommandQueueing Message-ID: <20040615113647.GJ25903@suse.de> References: <200406151202.12884.Guillaume@Lacote.name> Mime-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <200406151202.12884.Guillaume@Lacote.name> Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1503 Lines: 36 On Tue, Jun 15 2004, Guillaume Lac?te wrote: > Hello, > (I hope this is the right place for this - sorry if it is not). > > Native Command Queueing (and Tagged Command Queueing) is a feature > provided by the hardware of newer IDE (and old SCSI) disk drives which > basically consists in reordering the commands issued on the ATA bus to > improve speed. > > I assume however that the fastest way to read sectors 101 to 110 is to > ask for them in that order: 101,102,...,110 . This is a basic > assumption made by most OSes and apps I presume (otherwise for example > DMA performance would be catastrophic). > > Here is my point: since a bvec consists of _ordered_ requests only, > what is the use of NCQ ? Requests will arrive to the drive in > increasing order, which is the best possible ordering > performance-wise; thus NCQ will do never do anything. I think you are confusing scatter-gather with request ordering. And your terminology is off base - a bvec doesn't consist of ordered requests, it consist of (max) a single page. A bio consists of bvec's. A request consits of ordered bio's. The drive queue consist of (fairly well) ordered requests. I won't go on about merrits of queueing and depths, search the archives for that. -- Jens Axboe - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/