On Tue, Aug 30, 2022 at 03:30:13PM -0400, Jeff Layton wrote: > On Tue, 2022-08-30 at 14:32 -0400, J. Bruce Fields wrote: > > On Tue, Aug 30, 2022 at 01:02:50PM -0400, Jeff Layton wrote: > > > 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. > > > > Yeah, the afs change-attribute-as-counter thing seems ambitious--I > > wouldn't even know how to define what exactly you're counting. > > > > My one question is whether it'd be worth just defining the thing as > > *increasing*. That's a lower bar. > > > > That's a very good question. > > One could argue that NFSv4 sort of requires that for write delegations > anyway. All of the existing implementations that I know of do this, so > that wouldn't rule any of them out. > > I'm not opposed to adding that constraint. Let me think on it a bit > more. > > > (Though admittedly we don't quite manage it now--see again 1631087ba872 > > "Revert "nfsd4: support change_attr_type attribute"".) > > > > Factoring the ctime into the change attr seems wrong, since a clock jump > could make it go backward. Do you remember what drove that change (see > 630458e730b8) ? > > It seems like if the i_version were to go backward, then the ctime > probably would too, and you'd still see a duplicate change attr. See the comment--I was worried about crashes: the change attribute isn't on disk at the time the client requests it, so after a crash the client may see it go backward. (And then could see it repeat a value, possibly with different file contents.) Combining it with the ctime means we get something that behaves correctly even in that case--unless the clock goes backwards. --b.
