From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-2.2 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED,USER_AGENT_SANE_1 autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 55C43CA9EBD for ; Wed, 23 Oct 2019 22:14:25 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 2AF7121E6F for ; Wed, 23 Oct 2019 22:14:25 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S2436705AbfJWWNo (ORCPT ); Wed, 23 Oct 2019 18:13:44 -0400 Received: from mail104.syd.optusnet.com.au ([211.29.132.246]:39940 "EHLO mail104.syd.optusnet.com.au" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S2436687AbfJWWNo (ORCPT ); Wed, 23 Oct 2019 18:13:44 -0400 Received: from dread.disaster.area (pa49-180-40-48.pa.nsw.optusnet.com.au [49.180.40.48]) by mail104.syd.optusnet.com.au (Postfix) with ESMTPS id 00C6E43EE24; Thu, 24 Oct 2019 09:13:34 +1100 (AEDT) Received: from dave by dread.disaster.area with local (Exim 4.92.3) (envelope-from ) id 1iNOsm-0006fc-Rc; Thu, 24 Oct 2019 09:13:32 +1100 Date: Thu, 24 Oct 2019 09:13:32 +1100 From: Dave Chinner To: Boaz Harrosh Cc: ira.weiny@intel.com, linux-kernel@vger.kernel.org, Alexander Viro , "Darrick J. Wong" , Dan Williams , Christoph Hellwig , "Theodore Y. Ts'o" , Jan Kara , linux-ext4@vger.kernel.org, linux-xfs@vger.kernel.org, linux-fsdevel@vger.kernel.org Subject: Re: [PATCH 0/5] Enable per-file/directory DAX operations Message-ID: <20191023221332.GE2044@dread.disaster.area> References: <20191020155935.12297-1-ira.weiny@intel.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.10.1 (2018-07-13) X-Optus-CM-Score: 0 X-Optus-CM-Analysis: v=2.2 cv=D+Q3ErZj c=1 sm=1 tr=0 a=y881pOMu+B+mZdf5UrsJdA==:117 a=y881pOMu+B+mZdf5UrsJdA==:17 a=jpOVt7BSZ2e4Z31A5e1TngXxSK0=:19 a=kj9zAlcOel0A:10 a=XobE76Q3jBoA:10 a=QyXUC8HyAAAA:8 a=7-415B0cAAAA:8 a=iEe7G1TxEPlCt2B0xWcA:9 a=CjuIK1q_8ugA:10 a=biEYGPWJfzWAr4FL6Ov7:22 Sender: linux-fsdevel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-fsdevel@vger.kernel.org On Wed, Oct 23, 2019 at 04:09:50PM +0300, Boaz Harrosh wrote: > On 22/10/2019 14:21, Boaz Harrosh wrote: > > On 20/10/2019 18:59, ira.weiny@intel.com wrote: > Please explain the use case behind your model? No application changes needed to control whether they use DAX or not. It allows the admin to control the application behaviour completely, so they can turn off DAX if necessary. Applications are unaware of constraints that may prevent DAX from being used, and so admins need a mechanism to prevent DAX aware application from actually using DAX if the capability is present. e.g. given how slow some PMEM devices are when it comes to writing data, especially under extremely high concurrency, DAX is not necessarily a performance win for every application. Admins need a guaranteed method of turning off DAX in these situations - apps may not provide such a knob, or even be aware of a thing called DAX... e.g. the data set being accessed by the application is mapped and modified by RDMA applications, so those files must not be accessed using DAX by any application because DAX+RDMA are currently incompatible. Hence you can have RDMA on pmem devices co-exist within the same filesystem as other applications using DAX to access the pmem... Cheers, Dave. -- Dave Chinner david@fromorbit.com