Message-ID: <8b71a82b0dc0dbb121c9ef72f8a70389e729d831.camel@kernel.org>
Date:   Tue, 30 Aug 2022 15:11:46 -0400
From:   Jeff Layton <jlayton@...nel.org>
To:     Trond Myklebust <trondmy@...merspace.com>,
        "bfields@...ldses.org" <bfields@...ldses.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 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@...nel.org>

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