From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1762010AbYDXP4i (ORCPT ); Thu, 24 Apr 2008 11:56:38 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1759753AbYDXPzq (ORCPT ); Thu, 24 Apr 2008 11:55:46 -0400 Received: from brmea-mail-2.Sun.COM ([192.18.98.43]:49295 "EHLO brmea-mail-2.sun.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1759398AbYDXPzk (ORCPT ); Thu, 24 Apr 2008 11:55:40 -0400 Date: Thu, 24 Apr 2008 11:53:14 -0400 From: David Collier-Brown Subject: Re: [RFC][PATCH 0/3] Skip I/O merges when disabled In-reply-to: <20080424150425.GD12774@kernel.dk> To: Jens Axboe Cc: Andi Kleen , "Alan D. Brunelle" , "linux-kernel@vger.kernel.org" Reply-to: davecb@sun.com Message-id: <4810ACEA.5070401@sun.com> MIME-version: 1.0 Content-type: text/plain; format=flowed; charset=us-ascii Content-transfer-encoding: 7BIT X-Accept-Language: en-us, en References: <480F8936.5030406@hp.com> <87ve27gz4u.fsf@basil.nowhere.org> <67E36C56-E149-4C87-8788-05BA43C1C2AD@kernel.dk> <4810961A.4000104@firstfloor.org> <20080424150425.GD12774@kernel.dk> User-Agent: Mozilla/5.0 (X11; U; SunOS sun4u; en-US; rv:1.7) Gecko/20041221 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Jens Axboe wrote: > On Thu, Apr 24 2008, Andi Kleen wrote: > >>>Not a good idea IMHO, it's much better with an explicit setting. That >>>way you don't introduce indeterministic behavior. >> >>So you would be deterministically slower. > > > Yes, absolutely. Think about the case for a second - the potential gain is in > fractions of a percent basically, the potential loss however is HUGE. > There's absolutely no way on earth I'd ever make this dynamic. If this is intended for databases, it might be backwards (;-)) The commercial unix "forcedirectio" option that Oracle and other database vendors usually ask for turns out to be a benefit in large sequential data transfers, because it does two things: 1) transfers directly between user address space and disk, avoiding buffering, and 2) allows enthusiastic coalescence of synchronous writes Is this intended for DBMSs, or for something esle? --dave -- David Collier-Brown | Always do right. This will gratify Sun Microsystems, Toronto | some people and astonish the rest davecb@sun.com | -- Mark Twain (905) 943-1983, cell: (647) 833-9377, (800) 555-9786 x56583 bridge: (877) 385-4099 code: 506 9191#