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

[Jeff Layton <jlayton@kernel.org>]

On Tue, 2023-10-24 at 10:08 +0300, Amir Goldstein wrote:
> 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.
> 

This would certainly be my preference. I don't want to break any
existing users though.

Perhaps this ought to be a mkfs option? Existing XFS filesystems could
still behave with the legacy behavior, but we could make mkfs.xfs build
filesystems by default that work like NFS requires.

-- 
Jeff Layton <jlayton@kernel.org>