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@xxxxxxxxxxxxxxx/ > > > > > > 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@xxxxxxxxxx>
