Message-ID: <5fd1f7e99d5ab87db48c8c3603b014c1c2d2ec5a.camel@hammerspace.com>
Date:   Tue, 30 Aug 2022 17:47:05 +0000
From:   Trond Myklebust <trondmy@...merspace.com>
To:     "bfields@...ldses.org" <bfields@...ldses.org>,
        "jlayton@...nel.org" <jlayton@...nel.org>
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Subject: Re: [PATCH v3 1/7] iversion: update comments with info about atime
 updates

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.

-- 
Trond Myklebust
Linux NFS client maintainer, Hammerspace
trond.myklebust@...merspace.com

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