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=-2.3 required=3.0 tests=DKIM_INVALID,DKIM_SIGNED, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_PASS,USER_AGENT_MUTT 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 57B94C4360F for ; Wed, 27 Mar 2019 12:00:34 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 265792146E for ; Wed, 27 Mar 2019 12:00:34 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=fail reason="signature verification failed" (2048-bit key) header.d=infradead.org header.i=@infradead.org header.b="HBOGRXtW" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728860AbfC0MAc (ORCPT ); Wed, 27 Mar 2019 08:00:32 -0400 Received: from bombadil.infradead.org ([198.137.202.133]:55032 "EHLO bombadil.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726241AbfC0MAc (ORCPT ); Wed, 27 Mar 2019 08:00:32 -0400 DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=bombadil.20170209; h=In-Reply-To:Content-Type:MIME-Version :References:Message-ID:Subject:Cc:To:From:Date:Sender:Reply-To: Content-Transfer-Encoding:Content-ID:Content-Description:Resent-Date: Resent-From:Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID:List-Id: List-Help:List-Unsubscribe:List-Subscribe:List-Post:List-Owner:List-Archive; bh=8UwcpcqaP8ZhHswLLo1/ZfvVHoz/qtIhN4RZjhfBKN0=; b=HBOGRXtWom6XXXTXi/OhNRxA1 C0ISWjCZaaFZskeTvAGowoxsDQcW16ZEC5H+7ri6b4TpkbJRPWNf0rgwqw2qLWtiSLubyVMnfBpUo Id0BD3qqC3bEc/LpOo/cTLM0UMwogmQt8WwXLoSV2mOQasiRHV9zjdgSL9puByknBlu+x3za+LgQf NwRWF8uuQ0YmZChZt+7q0xl3CT5F2bIQd/NU3gJcVaU0Vt3U9GObom3ezm3523CN9skm0F0oNwGrA v6KHJCu+M+Ds2hVjV3IrZGfWv8i9nP7j6ma6SJQrIGF00CUSK9d9LV6OUyhiaeFIErEURy+o2otnQ hSRBZcw6A==; Received: from willy by bombadil.infradead.org with local (Exim 4.90_1 #2 (Red Hat Linux)) id 1h97EO-0004h7-0C; Wed, 27 Mar 2019 12:00:32 +0000 Date: Wed, 27 Mar 2019 05:00:31 -0700 From: Matthew Wilcox To: Goldwyn Rodrigues 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: <20190327120031.GR10344@bombadil.infradead.org> References: <20190326190301.32365-1-rgoldwyn@suse.de> <20190326190301.32365-2-rgoldwyn@suse.de> <20190326191001.GP10344@bombadil.infradead.org> <20190327110052.pvn5dag2xwy2tnkv@merlin> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190327110052.pvn5dag2xwy2tnkv@merlin> User-Agent: Mutt/1.9.2 (2017-12-15) Sender: linux-fsdevel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-fsdevel@vger.kernel.org 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. > > 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? > 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.