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

Amir Goldstein
2023-10-24T07:08:51Z

On Tue, Oct 24, 2023 at 6:40 AM Dave Chinner <david@fromorbit.com> wrote:
>
> On Mon, Oct 23, 2023 at 02:18:12PM -1000, Linus Torvalds wrote:
> > On Mon, 23 Oct 2023 at 13:26, Dave Chinner <david@fromorbit.com> wrote:
> > >
> > > The problem is the first read request after a modification has been
> > > made. That is causing relatime to see mtime > atime and triggering
> > > an atime update. XFS sees this, does an atime update, and in
> > > committing that persistent inode metadata update, it calls
> > > inode_maybe_inc_iversion(force = false) to check if an iversion
> > > update is necessary. The VFS sees I_VERSION_QUERIED, and so it bumps
> > > i_version and tells XFS to persist it.
> >
> > Could we perhaps just have a mode where we don't increment i_version
> > for just atime updates?
> >
> > Maybe we don't even need a mode, and could just decide that atime
> > updates aren't i_version updates at all?
>
> We do that already - in memory atime updates don't bump i_version at
> all. The issue is the rare persistent atime update requests that
> still happen - they are the ones that trigger an i_version bump on
> XFS, and one of the relatime heuristics tickle this specific issue.
>
> If we push the problematic persistent atime updates to be in-memory
> updates only, then the whole problem with i_version goes away....
>
> > Yes, yes, it's obviously technically a "inode modification", but does
> > anybody actually *want* atime updates with no actual other changes to
> > be version events?
>
> Well, yes, there was. That's why we defined i_version in the on disk
> format this way well over a decade ago. It was part of some deep
> dark magical HSM beans that allowed the application to combine
> multiple scans for different inode metadata changes into a single
> pass. atime changes was one of the things it needed to know about
> for tiering and space scavenging purposes....
>

But if this is such an ancient mystical program, why do we have to
keep this XFS behavior in the present?
BTW, is this the same HSM whose DMAPI ioctls were deprecated
a few years back?

I mean, I understand that you do not want to change the behavior of
i_version update without an opt-in config or mount option - let the distro
make that choice.
But calling this an "on-disk format change" is a very long stretch.

Does xfs_repair guarantee that changes of atime, or any inode changes
for that matter, update i_version? No, it does not.
So IMO, "atime does not update i_version" is not an "on-disk format change",
it is a runtime behavior change, just like lazytime is.

Thanks,
Amir.