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. 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. --b. > We don't have a good way to > do that in userland for now. > > This is another place where fsinfo() would have been nice to have. I > think until we have something like that, we'd want to keep our promises > to userland to a minimum. > > [1]: https://www.rfc-editor.org/rfc/rfc7862.html#section-12.2.3 . I > guess I should look at plumbing this in for IS_I_VERSION inodes... > > -- > Jeff Layton <jlayton@xxxxxxxxxx> > >
