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=-3.0 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_PASS,USER_AGENT_NEOMUTT autolearn=unavailable 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 E3889C10F00 for ; Wed, 27 Mar 2019 12:26:13 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id B29C72087C for ; Wed, 27 Mar 2019 12:26:13 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1729448AbfC0M0M (ORCPT ); Wed, 27 Mar 2019 08:26:12 -0400 Received: from mx2.suse.de ([195.135.220.15]:55546 "EHLO mx1.suse.de" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1728696AbfC0M0L (ORCPT ); Wed, 27 Mar 2019 08:26:11 -0400 X-Virus-Scanned: by amavisd-new at test-mx.suse.de Received: from relay2.suse.de (unknown [195.135.220.254]) by mx1.suse.de (Postfix) with ESMTP id AA26AAD0C; Wed, 27 Mar 2019 12:26:10 +0000 (UTC) Date: Wed, 27 Mar 2019 07:26:08 -0500 From: Goldwyn Rodrigues To: Matthew Wilcox Cc: linux-btrfs@vger.kernel.org, linux-fsdevel@vger.kernel.org Subject: Re: [PATCH 01/15] btrfs: create a mount option for dax Message-ID: <20190327122608.qa3z7h2wgnaiiugt@merlin> References: <20190326190301.32365-1-rgoldwyn@suse.de> <20190326190301.32365-2-rgoldwyn@suse.de> <20190326191001.GP10344@bombadil.infradead.org> <20190327110052.pvn5dag2xwy2tnkv@merlin> <20190327120031.GR10344@bombadil.infradead.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190327120031.GR10344@bombadil.infradead.org> User-Agent: NeoMutt/20180323 Sender: linux-fsdevel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-fsdevel@vger.kernel.org On 5:00 27/03, Matthew Wilcox wrote: > On Wed, Mar 27, 2019 at 06:00:52AM -0500, Goldwyn Rodrigues wrote: > > On 12:10 26/03, Matthew Wilcox wrote: > > > On Tue, Mar 26, 2019 at 02:02:47PM -0500, Goldwyn Rodrigues wrote: > > > > This sets S_DAX in inode->i_flags, which can be used with > > > > IS_DAX(). > > > > > > > > The dax option is restricted to non multi-device mounts. > > > > dax interacts with the device directly instead of using bio, so > > > > all bio-hooks which we use for multi-device cannot be performed > > > > here. While regular read/writes could be manipulated with > > > > RAID0/1, mmap() is still an issue. > > > > > > > > Auto-setting free space tree, because dealing with free space > > > > inode (specifically readpages) is a nightmare. > > > > Auto-setting nodatasum because we don't get callback for writing > > > > checksums after mmap()s. > > > > > > Congratulations on getting the bear to dance. But why? > > > > Why not ? ;) > > 18 files changed, 662 insertions(+), 77 deletions(-) > > I want to know what advantage we're getting for that. Direct device mmap'd files, faster access, lower pagecache usage... all the advantages you would get from a dax device. > > > > To me, the point of btrfs is all the cool stuff it does with built-in > > > checksumming and snapshots and RAID and so on. DAX doesn't let you do > > > any of that, so why would somebody want to use btrfs to manage DAX? > > > > There are users who are asking for advantages of dax on btrfs. > > There are also people asking for perpetual motion machines. Do these > users understand the tradeoffs? The only major feature they would not be able to use is multi-device, which will be relayed at mount time. > > > I have tried to make it work with snapshots in this series. > > Checksumming should be possible, but would require some more hacks. I am > > looking into it. > > multi-device is an issue for mmap() and I don't think we can work around > > it. > > > > I agree there is a price to pay to use dax, but I am sure the users > > would know about that. > > I really doubt it, to be honest. Care to elaborate? -- Goldwyn