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    SubjectRe: [PATCH v3 1/7] iversion: update comments with info about atime updates
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    On Tue, 2022-08-30 at 18:25 +0000, Trond Myklebust wrote:
    > On Tue, 2022-08-30 at 13:53 -0400, Jeff Layton wrote:
    > > On Tue, 2022-08-30 at 17:47 +0000, Trond Myklebust wrote:
    > > > On Tue, 2022-08-30 at 13:02 -0400, Jeff Layton wrote:
    > > > > On Tue, 2022-08-30 at 15:43 +0000, Trond Myklebust wrote:
    > > > > > On Tue, 2022-08-30 at 11:17 -0400, J. Bruce Fields wrote:
    > > > > > > On Tue, Aug 30, 2022 at 02:58:27PM +0000, Trond Myklebust
    > > > > > > wrote:
    > > > > > > > On Tue, 2022-08-30 at 10:44 -0400, J. Bruce Fields wrote:
    > > > > > > > > On Tue, Aug 30, 2022 at 09:50:02AM -0400, Jeff Layton
    > > > > > > > > wrote:
    > > > > > > > > > On Tue, 2022-08-30 at 09:24 -0400, J. Bruce Fields
    > > > > > > > > > wrote:
    > > > > > > > > > > On Tue, Aug 30, 2022 at 07:40:02AM -0400, Jeff Layton
    > > > > > > > > > > wrote:
    > > > > > > > > > > > Yes, saying only that it must be different is
    > > > > > > > > > > > intentional.
    > > > > > > > > > > > What
    > > > > > > > > > > > we
    > > > > > > > > > > > really want is for consumers to treat this as an
    > > > > > > > > > > > opaque
    > > > > > > > > > > > value
    > > > > > > > > > > > for the
    > > > > > > > > > > > most part [1]. Therefore an implementation based on
    > > > > > > > > > > > hashing
    > > > > > > > > > > > would
    > > > > > > > > > > > conform to the spec, I'd think, as long as all of
    > > > > > > > > > > > the
    > > > > > > > > > > > relevant
    > > > > > > > > > > > info is
    > > > > > > > > > > > part of the hash.
    > > > > > > > > > >
    > > > > > > > > > > It'd conform, but it might not be as useful as an
    > > > > > > > > > > increasing
    > > > > > > > > > > value.
    > > > > > > > > > >
    > > > > > > > > > > E.g. a client can use that to work out which of a
    > > > > > > > > > > series
    > > > > > > > > > > of
    > > > > > > > > > > reordered
    > > > > > > > > > > write replies is the most recent, and I seem to
    > > > > > > > > > > recall
    > > > > > > > > > > that
    > > > > > > > > > > can
    > > > > > > > > > > prevent
    > > > > > > > > > > unnecessary invalidations in some cases.
    > > > > > > > > > >
    > > > > > > > > >
    > > > > > > > > > That's a good point; the linux client does this. That
    > > > > > > > > > said,
    > > > > > > > > > NFSv4
    > > > > > > > > > has a
    > > > > > > > > > way for the server to advertise its change attribute
    > > > > > > > > > behavior
    > > > > > > > > > [1]
    > > > > > > > > > (though nfsd hasn't implemented this yet).
    > > > > > > > >
    > > > > > > > > It was implemented and reverted.  The issue was that I
    > > > > > > > > thought
    > > > > > > > > nfsd
    > > > > > > > > should mix in the ctime to prevent the change attribute
    > > > > > > > > going
    > > > > > > > > backwards
    > > > > > > > > on reboot (see fs/nfsd/nfsfh.h:nfsd4_change_attribute()),
    > > > > > > > > but
    > > > > > > > > Trond
    > > > > > > > > was
    > > > > > > > > concerned about the possibility of time going backwards. 
    > > > > > > > > See
    > > > > > > > > 1631087ba872 "Revert "nfsd4: support change_attr_type
    > > > > > > > > attribute"".
    > > > > > > > > There's some mailing list discussion to that I'm not
    > > > > > > > > turning
    > > > > > > > > up
    > > > > > > > > right
    > > > > > > > > now.
    > > > > > >
    > > > > > > https://lore.kernel.org/linux-nfs/a6294c25cb5eb98193f609a52aa8f4b5d4e81279.camel@hammerspace.com/
    > > > > > > is what I was thinking of but it isn't actually that
    > > > > > > interesting.
    > > > > > >
    > > > > > > > My main concern was that some filesystems (e.g. ext3) were
    > > > > > > > failing
    > > > > > > > to
    > > > > > > > provide sufficient timestamp resolution to actually label
    > > > > > > > the
    > > > > > > > resulting
    > > > > > > > 'change attribute' as being updated monotonically. If the
    > > > > > > > time
    > > > > > > > stamp
    > > > > > > > doesn't change when the file data or metadata are changed,
    > > > > > > > then
    > > > > > > > the
    > > > > > > > client has to perform extra checks to try to figure out
    > > > > > > > whether
    > > > > > > > or
    > > > > > > > not
    > > > > > > > its caches are up to date.
    > > > > > >
    > > > > > > That's a different issue from the one you were raising in
    > > > > > > that
    > > > > > > discussion.
    > > > > > >
    > > > > > > > > Did NFSv4 add change_attr_type because some
    > > > > > > > > implementations
    > > > > > > > > needed
    > > > > > > > > the
    > > > > > > > > unordered case, or because they realized ordering was
    > > > > > > > > useful
    > > > > > > > > but
    > > > > > > > > wanted
    > > > > > > > > to keep backwards compatibility?  I don't know which it
    > > > > > > > > was.
    > > > > > > >
    > > > > > > > We implemented it because, as implied above, knowledge of
    > > > > > > > whether
    > > > > > > > or
    > > > > > > > not the change attribute behaves monotonically, or strictly
    > > > > > > > monotonically, enables a number of optimisations.
    > > > > > >
    > > > > > > Of course, but my question was about the value of the old
    > > > > > > behavior,
    > > > > > > not
    > > > > > > about the value of the monotonic behavior.
    > > > > > >
    > > > > > > Put differently, if we could redesign the protocol from
    > > > > > > scratch
    > > > > > > would
    > > > > > > we
    > > > > > > actually have included the option of non-monotonic behavior?
    > > > > > >
    > > > > >
    > > > > > If we could design the filesystems from scratch, we probably
    > > > > > would
    > > > > > not.
    > > > > > The protocol ended up being as it is because people were trying
    > > > > > to
    > > > > > make
    > > > > > it as easy to implement as possible.
    > > > > >
    > > > > > So if we could design the filesystem from scratch, we would
    > > > > > have
    > > > > > probably designed it along the lines of what AFS does.
    > > > > > i.e. each explicit change is accompanied by a single bump of
    > > > > > the
    > > > > > change
    > > > > > attribute, so that the clients can not only decide the order of
    > > > > > the
    > > > > > resulting changes, but also if they have missed a change (that
    > > > > > might
    > > > > > have been made by a different client).
    > > > > >
    > > > > > However that would be a requirement that is likely to be very
    > > > > > specific
    > > > > > to distributed caches (and hence distributed filesystems). I
    > > > > > doubt
    > > > > > there are many user space applications that would need that
    > > > > > high
    > > > > > precision. Maybe MPI, but that's the only candidate I can think
    > > > > > of
    > > > > > for
    > > > > > now?
    > > > > >
    > > > >
    > > > > The fact that NFS kept this more loosely-defined is what allowed
    > > > > us
    > > > > to
    > > > > elide some of the i_version bumps and regain a fair bit of
    > > > > performance
    > > > > for local filesystems [1]. If the change attribute had been more
    > > > > strictly defined like you mention, then that particular
    > > > > optimization
    > > > > would not have been possible.
    > > > >
    > > > > This sort of thing is why I'm a fan of not defining this any more
    > > > > strictly than we require. Later on, maybe we'll come up with a
    > > > > way
    > > > > for
    > > > > filesystems to advertise that they can offer stronger guarantees.
    > > >
    > > > What 'eliding of the bumps' are we talking about here? If it
    > > > results in
    > > > unreliable behaviour, then I propose we just drop the whole concept
    > > > and
    > > > go back to using the ctime. The change attribute is only useful if
    > > > it
    > > > results in a reliable mechanism for detecting changes. Once you
    > > > "elide
    > > > away" the word "reliable", then it has no value beyond what ctime
    > > > already does.
    > > >
    > >
    > > I'm talking about the scheme to optimize away i_version updates when
    > > the
    > > current one has never been queried:
    > >
    > >    
    > > https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=f02a9ad1f15d
    > >
    > > There's nothing unreliable about it.
    >
    > Not really seeing why that would be incompatible with the idea of
    > bumping on every change. The I_VERSION_QUERIED is just a hint to tell
    > you that at the very least you need to sync the next metadata update
    > after someone peeked at the value. You could still continue to cache
    > updates after that, and only sync them once a O_SYNC or an fsync() call
    > explicitly requires you to do so.
    >

    Good point! It's not implemented that way today, but we could change it
    to do that if it were useful. I think it'd be slightly more costly
    CPU-wise when the update isn't going to disk, since you'd now have to
    update the value on every change instead of just skipping it, but I
    doubt anyone would notice the extra overhead.
    --
    Jeff Layton <jlayton@kernel.org>

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