From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S934170AbXC0Siz (ORCPT ); Tue, 27 Mar 2007 14:38:55 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S934169AbXC0Siz (ORCPT ); Tue, 27 Mar 2007 14:38:55 -0400 Received: from srv5.dvmed.net ([207.36.208.214]:38151 "EHLO mail.dvmed.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S934167AbXC0Six (ORCPT ); Tue, 27 Mar 2007 14:38:53 -0400 Message-ID: <460964BA.8090101@garzik.org> Date: Tue, 27 Mar 2007 14:38:50 -0400 From: Jeff Garzik User-Agent: Thunderbird 1.5.0.10 (X11/20070302) MIME-Version: 1.0 To: Mark Rustad CC: Justin Piszcz , linux-kernel@vger.kernel.org, IDE/ATA development list Subject: Re: Why is NCQ enabled by default by libata? (2.6.20) References: <4608B2B9.7090503@garzik.org> In-Reply-To: Content-Type: text/plain; charset=windows-1252; format=flowed Content-Transfer-Encoding: 8bit X-Spam-Score: -4.3 (----) X-Spam-Report: SpamAssassin version 3.1.8 on srv5.dvmed.net summary: Content analysis details: (-4.3 points, 5.0 required) Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org Mark Rustad wrote: > reorder any queued operations. Of course if you really care about your > data, you don't really want to turn write cache on. That's a gross exaggeration. FLUSH CACHE and FUA both ensure data integrity as well. Turning write cache off has always been a performance-killing action on ATA. > Also the controller used can have unfortunate interactions. For example > the Adaptec SAS controller firmware will never issue more than two > queued commands to a SATA drive (even though the firmware will happily > accept more from the driver), so even if an attached drive is capable of > reordering queued commands, its performance is seriously crippled by not > getting more commands queued up. In addition, some drive firmware seems > to try to bunch up queued command completions which interacts very badly > with a controller that queues up so few commands. In this case turning > NCQ off performs better because the drive knows it can't hold off > completions to reduce interrupt load on the host – a good idea gone > totally wrong when used with the Adaptec controller. All of that can be fixed with an Adaptec firmware upgrade, so not our problem here, and not a reason to disable NCQ in libata core. > Today SATA NCQ seems to be an area where few combinations work well. It > seems so bad to me that a whitelist might be better than a blacklist. > That is probably overstating it, but NCQ performance is certainly a big > problem. Real world testing disagrees with you. NCQ has been enabled for a while now. We would have screaming hordes of users if the majority of configurations were problematic. Jeff