Re: [PATCH RFC 2/9] timekeeping: new interfaces for multigrain timestamp handing

Amir Goldstein
2023-11-01T11:38:58Z

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.