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: Thu, 29 Mar 2007 14:51:34 -0400 Message-ID: <460C0AB6.9020709@garzik.org> References: <20070327161616.31448.qmail@science.horizon.com> <460A7EE7.5070101@cfl.rr.com> <460A8053.3040209@garzik.org> <460BF73C.5070707@cfl.rr.com> 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]:55525 "EHLO mail.dvmed.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S934311AbXC2Svw (ORCPT ); Thu, 29 Mar 2007 14:51:52 -0400 In-Reply-To: <460BF73C.5070707@cfl.rr.com> Sender: linux-ide-owner@vger.kernel.org List-Id: linux-ide@vger.kernel.org To: Phillip Susi Cc: Justin Piszcz , linux@horizon.com, htejun@gmail.com, linux-ide@vger.kernel.org, linux-kernel@vger.kernel.org Phillip Susi wrote: > Jeff Garzik wrote: >> NCQ provides for a more asynchronous flow. It helps greatly with >> reads (of which most are, by nature, synchronous at the app level) >> from multiple threads or apps. It helps with writes, even with write >> cache on, by allowing multiple commands to be submitted and/or retired >> at the same time. > > But when writing, what is the difference between queuing multiple tagged > writes, and sending down multiple untagged cached writes that complete > immediately and actually hit the disk later? Either way the host keeps > sending writes to the disk until it's buffers are full, and the disk is > constantly trying to commit those buffers to the media in the most > optimal order. Less overhead to starting commands, and all the other benefits of making operations fully async. Jeff