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