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=-7.2 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,NICE_REPLY_A,SPF_HELO_NONE, SPF_PASS,UNPARSEABLE_RELAY,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 CD671C432BE for ; Tue, 17 Aug 2021 13:23:03 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id B72A160F35 for ; Tue, 17 Aug 2021 13:23:03 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S236950AbhHQNXf (ORCPT ); Tue, 17 Aug 2021 09:23:35 -0400 Received: from out30-131.freemail.mail.aliyun.com ([115.124.30.131]:54174 "EHLO out30-131.freemail.mail.aliyun.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S235603AbhHQNX2 (ORCPT ); Tue, 17 Aug 2021 09:23:28 -0400 X-Alimail-AntiSpam: AC=PASS;BC=-1|-1;BR=01201311R381e4;CH=green;DM=||false|;DS=||;FP=0|-1|-1|-1|0|-1|-1|-1;HT=e01e04423;MF=jefflexu@linux.alibaba.com;NM=1;PH=DS;RN=8;SR=0;TI=SMTPD_---0UjPoNDD_1629206573; Received: from admindeMacBook-Pro-2.local(mailfrom:jefflexu@linux.alibaba.com fp:SMTPD_---0UjPoNDD_1629206573) by smtp.aliyun-inc.com(127.0.0.1); Tue, 17 Aug 2021 21:22:53 +0800 Subject: Re: [PATCH v4 0/8] fuse,virtiofs: support per-file DAX To: Vivek Goyal , Miklos Szeredi Cc: Stefan Hajnoczi , linux-fsdevel@vger.kernel.org, virtualization@lists.linux-foundation.org, virtio-fs-list , Joseph Qi , Liu Bo References: <20210817022220.17574-1-jefflexu@linux.alibaba.com> From: JeffleXu Message-ID: <6043c0b8-0ff1-2e11-0dd0-e23f9ff6b952@linux.alibaba.com> Date: Tue, 17 Aug 2021 21:22:53 +0800 User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.15; rv:78.0) Gecko/20100101 Thunderbird/78.11.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-fsdevel@vger.kernel.org On 8/17/21 8:39 PM, Vivek Goyal wrote: > On Tue, Aug 17, 2021 at 10:06:53AM +0200, Miklos Szeredi wrote: >> On Tue, 17 Aug 2021 at 04:22, Jeffle Xu wrote: >>> >>> This patchset adds support of per-file DAX for virtiofs, which is >>> inspired by Ira Weiny's work on ext4[1] and xfs[2]. >> >> Can you please explain the background of this change in detail? >> >> Why would an admin want to enable DAX for a particular virtiofs file >> and not for others? > > Initially I thought that they needed it because they are downloading > files on the fly from server. So they don't want to enable dax on the file > till file is completely downloaded. Right, it's our initial requirement. > But later I realized that they should > be able to block in FUSE_SETUPMAPPING call and make sure associated > file section has been downloaded before returning and solve the problem. > So that can't be the primary reason. Saying we want to access 4KB of one file inside guest, if it goes through FUSE request routine, then the fuse daemon only need to download this 4KB from remote server. But if it goes through DAX, then the fuse daemon need to download the whole DAX window (e.g., 2MB) from remote server, so called amplification. Maybe we could decrease the DAX window size, but it's a trade off. > > Other reason mentioned I think was that only certain files benefit > from DAX. But not much details are there after that. It will be nice > to hear a more concrete use case and more details about this usage. > Apart from our internal requirement, more fine grained control for DAX shall be general and more flexible. Glad to hear more discussion from community. -- Thanks, Jeffle