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 Received: from bombadil.infradead.org (bombadil.infradead.org [198.137.202.133]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id 1BDFDCE79D7 for ; Wed, 20 Sep 2023 11:57:05 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=lists.infradead.org; s=bombadil.20210309; h=Sender: Content-Transfer-Encoding:Content-Type:List-Subscribe:List-Help:List-Post: List-Archive:List-Unsubscribe:List-Id:MIME-Version:References:In-Reply-To: Date:Cc:To:From:Subject:Message-ID:Reply-To:Content-ID:Content-Description: Resent-Date:Resent-From:Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID: List-Owner; bh=YNMo3PjHt/8GNKxQUpuZLMcJaKuol8ixbrx8fzd9fFk=; b=CyC8DOg65EpKvf hNXlYgnJnR8Od++szKwGX3i1qWUXFu2TrOdYF7wrGzgQcJn6nzsSx8l/XpPhLzzMtKDRTbMNAexLA xEbnRrmwMpHGZkmmrXkpIg8+pSkA9lK67YSnUUJVGaUrWwwJpEeprB2UBEHClX9DRFOkg+vDmbiSB pR+IW2nUvWBIbZhv4FEIKHofbhFklNORslrCjxUy4ItAdf5/cFcavVAEH/jInKCsljCPAl6mT/6vr YR16F8mB875569k5TTx9D2BSu6nA17Py4B78xWIDf8HpbLJTVVpY9ewOupOCCdlxlQ5PTTNER1tyj 4AZ5cAbQgG7BwyO+BGzA==; Received: from localhost ([::1] helo=bombadil.infradead.org) by bombadil.infradead.org with esmtp (Exim 4.96 #2 (Red Hat Linux)) id 1qivpC-002wOQ-0S; Wed, 20 Sep 2023 11:56:58 +0000 Received: from ams.source.kernel.org ([2604:1380:4601:e00::1]) by bombadil.infradead.org with esmtps (Exim 4.96 #2 (Red Hat Linux)) id 1qivp8-002wM0-0o; Wed, 20 Sep 2023 11:56:56 +0000 Received: from smtp.kernel.org (relay.kernel.org [52.25.139.140]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (2048 bits)) (No client certificate requested) by ams.source.kernel.org (Postfix) with ESMTPS id 877BDB81BEB; Wed, 20 Sep 2023 11:56:52 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 677F3C433CD; Wed, 20 Sep 2023 11:56:44 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1695211011; bh=v+kxoG9eOGJ5xusIWXasOkiwnORCBCOMs4NdoCLCV+Y=; h=Subject:From:To:Cc:Date:In-Reply-To:References:From; b=Rg4SDYT2G/5iLIMjOccnvEi1JiFZ6DWISJeHQBkraG1wlEzFngIZ40YiKunM8cbSc +aGiW2BG1XlefCOYsICZDxSIj/cycJoqoVgH21scC+LvhZuM/I7QY33SH4s37GiB3i xx8VoYkq8098drZ1/id4lmBEMu3eQVNXWTxAUaEp2MAhsi3wNH08SmNvjfcYqsJCu1 KXrrrvTysK40Yb4V8RFHdyvz2X1AZgX3gj24E21SykmhPZrGV8Spjbs29Y+Wz+E6KX beC0Mxqh0EOjy/WL/hLY5L03MPDQWMARcGn8Qnl4hdNKDEe2Is8D1IwBJNuuGtXBBF rmhEIbbPeuCGA== Message-ID: <35c28758a9cc28a276a6b4b4ae8a420a1444e711.camel@kernel.org> Subject: Re: [PATCH v7 12/13] ext4: switch to multigrain timestamps From: Jeff Layton To: Christian Brauner Cc: Jan Kara , Bruno Haible , Xi Ruoyao , bug-gnulib@gnu.org, Alexander Viro , Eric Van Hensbergen , Latchesar Ionkov , Dominique Martinet , Christian Schoenebeck , David Howells , Marc Dionne , Chris Mason , Josef Bacik , David Sterba , Xiubo Li , Ilya Dryomov , Jan Harkes , coda@cs.cmu.edu, Tyler Hicks , Gao Xiang , Chao Yu , Yue Hu , Jeffle Xu , Namjae Jeon , Sungjong Seo , Jan Kara , Theodore Ts'o , Andreas Dilger , Jaegeuk Kim , OGAWA Hirofumi , Miklos Szeredi , Bo b Peterson , Andreas Gruenbacher , Greg Kroah-Hartman , Tejun Heo , Trond Myklebust , Anna Schumaker , Konstantin Komarov , Mark Fasheh , Joel Becker , Joseph Qi , Mike Marshall , Martin Brandenburg , Luis Chamberlain , Kees Cook , Iurii Zaikin , Steve French , Paulo Alcantara , Ronnie Sahlberg , Shyam Prasad N , Tom Talpey , Sergey Senozhatsky , Richard Weinberger , Hans de Goede , Hugh Dickins , Andrew Morton , Amir Goldstein , "Darrick J. Wong" , Benjamin Coddington , linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, v9fs@lists.linux.dev, linux-afs@lists.infradead.org, linux-btrfs@vger.kernel.org, ceph-devel@vger.kernel.org, codalist@coda.cs.cmu.edu, ecryptfs@vger.kernel.org, linux-erofs@lists.ozlabs.org, linux-ext4@vger.kernel.org, linux-f2fs-devel@lists.sourceforge.net, cluster-devel@redhat.com, linux-nfs@vger.kernel.org, ntfs3@lists.linux.dev, ocfs2-devel@lists.linux.dev, devel@lists.orangefs.org, linux-cifs@vger.kernel.org, samba-technical@lists.samba.org, linux-mtd@lists.infradead.org, linux-mm@kvack.org, linux-unionfs@vger.kernel.org, linux-xfs@vger.kernel.org Date: Wed, 20 Sep 2023 07:56:43 -0400 In-Reply-To: <20230920-raser-teehaus-029cafd5a6e4@brauner> References: <20230807-mgctime-v7-0-d1dec143a704@kernel.org> <20230919110457.7fnmzo4nqsi43yqq@quack3> <1f29102c09c60661758c5376018eac43f774c462.camel@kernel.org> <4511209.uG2h0Jr0uP@nimes> <08b5c6fd3b08b87fa564bb562d89381dd4e05b6a.camel@kernel.org> <20230920-leerung-krokodil-52ec6cb44707@brauner> <20230920101731.ym6pahcvkl57guto@quack3> <317d84b1b909b6c6519a2406fcb302ce22dafa41.camel@kernel.org> <20230920-raser-teehaus-029cafd5a6e4@brauner> User-Agent: Evolution 3.48.4 (3.48.4-1.fc38) MIME-Version: 1.0 X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20230920_045654_584408_08A3D2FD X-CRM114-Status: GOOD ( 33.68 ) X-BeenThere: linux-mtd@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: Linux MTD discussion mailing list List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Sender: "linux-mtd" Errors-To: linux-mtd-bounces+linux-mtd=archiver.kernel.org@lists.infradead.org On Wed, 2023-09-20 at 13:48 +0200, Christian Brauner wrote: > > > > While we initially thought we can do this unconditionally it turns out > > > > that this might break existing workloads that rely on timestamps in very > > > > specific ways and we always knew this was a possibility. Move > > > > multi-grain timestamps behind a vfs mount option. > > > > > > Surely this is a safe choice as it moves the responsibility to the sysadmin > > > and the cases where finegrained timestamps are required. But I kind of > > > wonder how is the sysadmin going to decide whether mgtime is safe for his > > > system or not? Because the possible breakage needn't be obvious at the > > > first sight... > > > > > > > That's the main reason I really didn't want to go with a mount option. > > Documenting that may be difficult. While there is some pessimism around > > it, I may still take a stab at just advancing the coarse clock whenever > > we fetch a fine-grained timestamp. It'd be nice to remove this option in > > the future if that turns out to be feasible. > > > > > If I were a sysadmin, I'd rather opt for something like > > > finegrained timestamps + lazytime (if I needed the finegrained timestamps > > > functionality). That should avoid the IO overhead of finegrained timestamps > > > as well and I'd know I can have problems with timestamps only after a > > > system crash. > > > > > I've just got another idea how we could solve the problem: Couldn't we > > > always just report coarsegrained timestamp to userspace and provide access > > > to finegrained value only to NFS which should know what it's doing? > > > > > > > I think that'd be hard. First of all, where would we store the second > > timestamp? We can't just truncate the fine-grained ones to come up with > > a coarse-grained one. It might also be confusing having nfsd and local > > filesystems present different attributes. > > As far as I can tell we have two options. The first one is to make this > into a mount option which I really think isn't a big deal and lets us > avoid this whole problem while allowing filesytems exposed via NFS to > make use of this feature for change tracking. > > The second option is that we turn off fine-grained finestamps for v6.6 > and you get to explore other options. > > It isn't a big deal regressions like this were always to be expected but > v6.6 needs to stabilize so anything that requires more significant work > is not an option. Oh, absolutely. I wasn't proposing to do that work for v6.6. For that, we absolutely either need the mount option or to just revert the mgtime conversions. My plan was to take a stab at doing this for a later kernel release. This is very much a "back to the drawing board" idea. It may not pan out after all, but if it does then we could consider removing the mount option at that point. -- Jeff Layton ______________________________________________________ Linux MTD discussion mailing list http://lists.infradead.org/mailman/listinfo/linux-mtd/