From: Amir Goldstein <amir73il@gmail.com>
To: Jan Kara <jack@suse.cz>
Cc: Dave Chinner <david@fromorbit.com>,
Jeff Layton <jlayton@kernel.org>,
Linus Torvalds <torvalds@linux-foundation.org>,
Kent Overstreet <kent.overstreet@linux.dev>,
Christian Brauner <brauner@kernel.org>,
Alexander Viro <viro@zeniv.linux.org.uk>,
John Stultz <jstultz@google.com>,
Thomas Gleixner <tglx@linutronix.de>,
Stephen Boyd <sboyd@kernel.org>,
Chandan Babu R <chandan.babu@oracle.com>,
"Darrick J. Wong" <djwong@kernel.org>,
"Theodore Ts'o" <tytso@mit.edu>,
Andreas Dilger <adilger.kernel@dilger.ca>,
Chris Mason <clm@fb.com>, Josef Bacik <josef@toxicpanda.com>,
David Sterba <dsterba@suse.com>, Hugh Dickins <hughd@google.com>,
Andrew Morton <akpm@linux-foundation.org>,
Jan Kara <jack@suse.de>, David Howells <dhowells@redhat.com>,
linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org,
linux-xfs@vger.kernel.org, linux-ext4@vger.kernel.org,
linux-btrfs@vger.kernel.org, linux-mm@kvack.org,
linux-nfs@vger.kernel.org
Subject: Re: [PATCH RFC 2/9] timekeeping: new interfaces for multigrain timestamp handing
Date: Wed, 1 Nov 2023 13:38:41 +0200 [thread overview]
Message-ID: <CAOQ4uxgGxtErFEcSdxoFDnZZ1XfmVKn2LT1dQcJqhNj5_rnC6A@mail.gmail.com> (raw)
In-Reply-To: <20231101101648.zjloqo5su6bbxzff@quack3>
On Wed, Nov 1, 2023 at 12:16 PM Jan Kara <jack@suse.cz> wrote:
>
> On Wed 01-11-23 08:57:09, Dave Chinner wrote:
> > 5. When-ever the inode is persisted, the timestamp is copied to the
> > on-disk structure and the current change counter is folded in.
> >
> > This means the on-disk structure always contains the latest
> > change attribute that has been persisted, just like we
> > currently do with i_version now.
> >
> > 6. When-ever we read the inode off disk, we split the change counter
> > from the timestamp and update the appropriate internal structures
> > with this information.
> >
> > This ensures that the VFS and userspace never see the change
> > counter implementation in the inode timestamps.
>
> OK, but is this compatible with the current XFS behavior? AFAICS currently
> XFS sets sb->s_time_gran to 1 so timestamps currently stored on disk will
> have some mostly random garbage in low bits of the ctime. Now if you look
> at such inode with a kernel using this new scheme, stat(2) will report
> ctime with low bits zeroed-out so if the ctime fetched in the old kernel was
> stored in some external database and compared to the newly fetched ctime, it
> will appear that ctime has gone backwards... Maybe we don't care but it is
> a user visible change that can potentially confuse something.
See xfs_inode_has_bigtime() and auto-upgrade of inode format in
xfs_inode_item_precommit().
In the case of BIGTIME inode format, admin needs to opt-in to
BIGTIME format conversion by setting an INCOMPAT_BIGTIME
sb feature flag.
I imagine that something similar (inode flag + sb flag) would need
to be done for the versioned-timestamp, but I think that in that case,
the feature flag could be RO_COMPAT - there is no harm in exposing
made-up nsec lower bits if fs would be mounted read-only on an old
kernel, is there?
The same RO_COMPAT feature flag could also be used to determine
s_time_gran, because IIUC, s_time_gran for timestamp updates
is uniform across all inodes.
I know that Dave said he wants to avoid changing on-disk format,
but I am hoping that this well defined and backward compat with
lazy upgrade, on-disk format change may be acceptable?
Thanks,
Amir.
next prev parent reply other threads:[~2023-11-01 11:38 UTC|newest]
Thread overview: 70+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-10-18 17:41 [PATCH RFC 0/9] fs: multigrain timestamps (redux) Jeff Layton
2023-10-18 17:41 ` [PATCH RFC 1/9] fs: switch timespec64 fields in inode to discrete integers Jeff Layton
2023-10-18 17:41 ` [PATCH RFC 2/9] timekeeping: new interfaces for multigrain timestamp handing Jeff Layton
2023-10-18 19:18 ` Linus Torvalds
2023-10-18 20:47 ` Jeff Layton
2023-10-18 21:31 ` Linus Torvalds
2023-10-18 21:52 ` Jeff Layton
2023-10-19 9:29 ` Christian Brauner
2023-10-19 11:28 ` Jeff Layton
2023-10-19 22:02 ` Dave Chinner
2023-10-20 12:12 ` Jeff Layton
2023-10-20 20:06 ` Linus Torvalds
2023-10-20 20:20 ` Linus Torvalds
2023-10-20 21:05 ` Jeff Layton
2023-10-22 22:17 ` Dave Chinner
2023-10-23 14:45 ` Jeff Layton
2023-10-23 23:26 ` Dave Chinner
2023-10-24 0:18 ` Linus Torvalds
2023-10-24 3:40 ` Dave Chinner
2023-10-24 4:10 ` Linus Torvalds
2023-10-24 7:08 ` Amir Goldstein
2023-10-24 18:40 ` Jeff Layton
2023-10-25 8:05 ` Dave Chinner
2023-10-25 10:41 ` Amir Goldstein
2023-10-25 12:25 ` Jeff Layton
2023-10-26 2:20 ` Dave Chinner
2023-10-26 5:42 ` Amir Goldstein
2023-10-27 10:35 ` Jeff Layton
2023-10-30 22:37 ` Dave Chinner
2023-10-30 23:11 ` Linus Torvalds
2023-10-31 1:42 ` Dave Chinner
2023-10-31 7:03 ` Amir Goldstein
2023-10-31 10:30 ` Christian Brauner
2023-10-31 11:29 ` Jeff Layton
2023-10-31 21:57 ` Dave Chinner
2023-10-31 23:02 ` Darrick J. Wong
2023-10-31 23:47 ` Dave Chinner
2023-11-01 10:16 ` Jan Kara
2023-11-01 11:38 ` Amir Goldstein [this message]
2023-11-02 10:17 ` Jeff Layton
2023-11-01 20:10 ` Linus Torvalds
2023-11-01 21:34 ` Trond Myklebust
2023-11-01 22:23 ` Linus Torvalds
2023-11-01 22:45 ` Trond Myklebust
2023-11-01 23:29 ` Dave Chinner
2023-11-02 10:29 ` Jeff Layton
2023-11-02 10:15 ` Jeff Layton
2023-10-31 23:12 ` Darrick J. Wong
2023-11-01 8:08 ` Amir Goldstein
2023-10-31 11:26 ` Jeff Layton
2023-10-31 19:43 ` John Stoffel
2023-10-31 11:04 ` Jeff Layton
2023-10-31 12:22 ` Jan Kara
2023-10-31 12:55 ` Jeff Layton
2023-10-30 23:34 ` ronnie sahlberg
2023-10-24 14:24 ` Jeff Layton
2023-10-24 19:06 ` Jeff Layton
2023-10-24 19:40 ` Linus Torvalds
2023-10-24 20:19 ` Jeff Layton
2023-10-31 10:26 ` Christian Brauner
2023-10-31 13:55 ` Jeff Layton
2023-10-19 22:00 ` Thomas Gleixner
2023-10-19 22:41 ` Jeff Layton
2023-10-18 17:41 ` [PATCH RFC 3/9] timekeeping: add new debugfs file to count multigrain timestamps Jeff Layton
2023-10-18 17:41 ` [PATCH RFC 4/9] fs: add infrastructure for " Jeff Layton
2023-10-18 17:41 ` [PATCH RFC 5/9] fs: have setattr_copy handle multigrain timestamps appropriately Jeff Layton
2023-10-18 17:41 ` [PATCH RFC 6/9] xfs: switch to multigrain timestamps Jeff Layton
2023-10-18 17:41 ` [PATCH RFC 7/9] ext4: " Jeff Layton
2023-10-18 17:41 ` [PATCH RFC 8/9] btrfs: convert " Jeff Layton
2023-10-18 17:41 ` [PATCH RFC 9/9] tmpfs: add support for " Jeff Layton
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