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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) Message-ID: <35c28758a9cc28a276a6b4b4ae8a420a1444e711.camel@kernel.org> From: Jeff Layton To: Christian Brauner 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-Mimecast-Impersonation-Protect: Policy=CLT - Impersonation Protection Definition; Similar Internal Domain=false; Similar Monitored External Domain=false; Custom External Domain=false; Mimecast External Domain=false; Newly Observed Domain=false; Internal User Name=false; Custom Display Name List=false; Reply-to Address Mismatch=false; Targeted Threat Dictionary=false; Mimecast Threat Dictionary=false; Custom Threat Dictionary=false X-Scanned-By: MIMEDefang 3.1 on 10.11.54.8 Subject: Re: [Cluster-devel] [PATCH v7 12/13] ext4: switch to multigrain timestamps X-BeenThere: cluster-devel@redhat.com X-Mailman-Version: 2.1.29 Precedence: list List-Id: "\[Cluster devel\]" List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Latchesar Ionkov , Martin Brandenburg , Konstantin Komarov , Jan Kara , linux-xfs@vger.kernel.org, "Darrick J. Wong" , Dominique Martinet , Christian Schoenebeck , linux-unionfs@vger.kernel.org, David Howells , Chris Mason , Andreas Dilger , Hans de Goede , Marc Dionne , codalist@coda.cs.cmu.edu, linux-afs@lists.infradead.org, linux-mtd@lists.infradead.org, Mike Marshall , Paulo Alcantara , Amir Goldstein , Eric Van Hensbergen , bug-gnulib@gnu.org, Miklos Szeredi , Richard Weinberger , Mark Fasheh , Hugh Dickins , Tyler Hicks , cluster-devel@redhat.com, coda@cs.cmu.edu, linux-mm@kvack.org, Gao Xiang , Iurii Zaikin , Namjae Jeon , Trond Myklebust , Xi Ruoyao , Shyam Prasad N , ecryptfs@vger.kernel.org, Kees Cook , ocfs2-devel@lists.linux.dev, linux-cifs@vger.kernel.org, Chao Yu , linux-erofs@lists.ozlabs.org, Josef Bacik , Tom Talpey , Tejun Heo , Yue Hu , Alexander Viro , Ronnie Sahlberg , David Sterba , Jaegeuk Kim , ceph-devel@vger.kernel.org, Xiubo Li , Ilya Dryomov , OGAWA Hirofumi , Jan Harkes , linux-nfs@vger.kernel.org, linux-ext4@vger.kernel.org, Theodore Ts'o , Joseph Qi , Greg Kroah-Hartman , v9fs@lists.linux.dev, ntfs3@lists.linux.dev, samba-technical@lists.samba.org, linux-kernel@vger.kernel.org, linux-f2fs-devel@lists.sourceforge.net, Steve French , Sergey Senozhatsky , Luis Chamberlain , Jeffle Xu , devel@lists.orangefs.org, Anna Schumaker , Jan Kara , linux-fsdevel@vger.kernel.org, Andrew Morton , Sungjong Seo , Bruno Haible , linux-btrfs@vger.kernel.org, Joel Becker Errors-To: cluster-devel-bounces@redhat.com Sender: "Cluster-devel" X-Scanned-By: MIMEDefang 3.1 on 10.11.54.3 X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: kernel.org Content-Type: text/plain; charset="ISO-8859-15" Content-Transfer-Encoding: quoted-printable 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