Re: [PATCH v8 0/5] fs: multigrain timestamps for XFS's change_cookie

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On Tue, Sep 26, 2023 at 07:31:55AM -0400, Jeff Layton wrote:
> On Tue, 2023-09-26 at 08:32 +1000, Dave Chinner wrote:
> > We also must not lose sight of the fact that the lazytime mount
> > option makes atime updates on XFS behave exactly as the nfsd/NFS
> > client application wants. That is, XFS will do in-memory atime
> > updates unless the atime update also sets S_VERSION to explicitly
> > bump the i_version counter if required. That leads to another
> > potential nfsd specific solution without requiring filesystems to
> > change on disk formats: the nfsd explicitly asks operations for lazy
> > atime updates...
> > 
> Not exactly. The problem with XFS's i_version is that it also bumps it
> on atime updates. lazytime reduces the number of atime updates to
> ~1/day. To be exactly what nfsd wants, you'd need to make that 0.

As long as there are future modifications going to those files,
lazytime completely elides the visibility of atime updates as they
get silently aggregated into future modifications and so there are
0 i_version changes as a resutl of pure atime updates in those cases.

If there are no future modifications, then just like relatime, there
is a timestamp update every 24hrs. That's no big deal, nobody is
complaining about this being a problem.

It's the "persistent atime update after modification" heuristic
implemented by relatime that is causing all the problems here. If
that behaviour is elided on the server side, then most of the client
side invalidation problems with these workloads go away.

IOWs, nfsd needs direct control over how atime updates should be
treated by the VFS/filesystem (i.e. as pure in-memory updates)
rather than leaving it to some heuristic that may do the exact
opposite of what the nfsd application needs.

That's the point I was making: we have emerging requirements for
per-operation timestamp update behaviour control with io_uring and
other non-blocking applications. The nfsd application also has
specific semantics it wants the VFS/filesystem to implement
(non-persistent atime unless something else changes)....

My point is that we've now failed a couple of times now to implement
what NFSD requires via trying to change VFS and/or filesystem
infrastructure to provide i_version or ctime semantics the nfsd
requires. That's a fairly good sign that we might not be approaching
this problem from the right direction, and so doubling down and
considering changing the timestamp infrastructure from the ground up
just to solve a relatively niche, filesystem specific issue doesn't
seem like the best approach.

OTOH, having the application actually tell the timestamp updates
exactly what semantics it needs (non blocking, persistent vs in
memory, etc) will allow the VFS and filesystems can do the right
thing for the application without having to worry about general
heuristics that sometimes do exactly the wrong thing....

Dave Chinner

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