From mboxrd@z Thu Jan 1 00:00:00 1970 From: Jeff Garzik Subject: Re: Why is NCQ enabled by default by libata? (2.6.20) Date: Tue, 27 Mar 2007 01:59:21 -0400 Message-ID: <4608B2B9.7090503@garzik.org> References: Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Return-path: Received: from srv5.dvmed.net ([207.36.208.214]:33172 "EHLO mail.dvmed.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S934057AbXC0F7X (ORCPT ); Tue, 27 Mar 2007 01:59:23 -0400 In-Reply-To: Sender: linux-ide-owner@vger.kernel.org List-Id: linux-ide@vger.kernel.org To: Justin Piszcz Cc: linux-kernel@vger.kernel.org, IDE/ATA development list Justin Piszcz wrote: > Without NCQ, performance is MUCH better on almost every operation, with > the exception of 2-3 items. Variables to take into account: * the drive (NCQ performance wildly varies) * the IO scheduler * the filesystem (if not measuring direct to blkdev) * application workload (or in your case, benchmark tool) * in particular, the threaded-ness of the apps For the overwhelming majority of combinations, NCQ should not /hurt/ performance. For the majority of combinations, NCQ helps (though it may not be often that you use more than 4-8 tags). In some cases, NCQ firmware may be broken. There is a Maxtor firmware id, and some Hitachi ids that people are leaning towards recommending be added to the libata 'horkage' list. Jeff