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 vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 728C0CE79D4 for ; Wed, 20 Sep 2023 11:57:04 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S234964AbjITL5H (ORCPT ); Wed, 20 Sep 2023 07:57:07 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:56088 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S234884AbjITL5E (ORCPT ); Wed, 20 Sep 2023 07:57:04 -0400 Received: from smtp.kernel.org (relay.kernel.org [52.25.139.140]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 62E1C119; Wed, 20 Sep 2023 04:56:51 -0700 (PDT) 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> Content-Type: text/plain; charset="ISO-8859-15" Content-Transfer-Encoding: quoted-printable User-Agent: Evolution 3.48.4 (3.48.4-1.fc38) MIME-Version: 1.0 Precedence: bulk List-ID: X-Mailing-List: linux-fsdevel@vger.kernel.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. > > >=20 > > > Surely this is a safe choice as it moves the responsibility to the sy= sadmin > > > and the cases where finegrained timestamps are required. But I kind o= f > > > 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 th= e > > > first sight... > > >=20 > >=20 > > 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 i= n > > the future if that turns out to be feasible. > >=20 > > > If I were a sysadmin, I'd rather opt for something like > > > finegrained timestamps + lazytime (if I needed the finegrained timest= amps > > > functionality). That should avoid the IO overhead of finegrained time= stamps > > > as well and I'd know I can have problems with timestamps only after a > > > system crash. > >=20 > > > I've just got another idea how we could solve the problem: Couldn't w= e > > > always just report coarsegrained timestamp to userspace and provide a= ccess > > > to finegrained value only to NFS which should know what it's doing? > > >=20 > >=20 > > 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. >=20 > 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. >=20 > The second option is that we turn off fine-grained finestamps for v6.6 > and you get to explore other options. >=20 > 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. --=20 Jeff Layton