Yea, got it now. That all makes sense. Thanks! Apologies for the noise. Now I just have to go and fix a bunch of our user's code so I can forget about negative lookups again... Daire On Thu, 1 Sept 2022 at 16:43, Trond Myklebust <trondmy@xxxxxxxxxxxxxxx> wrote: > > On Thu, 2022-09-01 at 16:27 +0100, Daire Byrne wrote: > > On Thu, 1 Sept 2022 at 14:55, Trond Myklebust > > <trondmy@xxxxxxxxxxxxxxx> wrote: > > > man 5 nfs > > > > > > Look for the section on the 'lookupcache=mode' mount option. > > > > So I get that the client caches negative lookups once we've made them > > (the default lookupcache=all), but what I'm wondering is if we have > > already cached the entire directory contents before the (negative) > > lookup, can we not reply that it doesn't exist using that information > > without having to go across the wire the at all (even the first > > time)? > > > > Or is there no concept of "cached directory contents"? I thought that > > maybe readdir/readdirplus knew about the "full contents" of a > > directory? > > > > My thinking was that if we did a readdir/readirplus first, we could > > then do lookups for any random non-existent filename without having > > to > > send anything across the wire. Like I said, a newbie question with > > limited understanding of the actual internals :) > > > > Daire > > There is no concept of a 'fully cached directory'. The VFS and the > memory management code are free to kick out any unused cached entries > from the dcache at any time and for any reason. So the absence of an > entry is not the same as a negative entry. > > Furthermore, certain features like case insensitive filesystems on > servers makes it hard for the NFS client to know whether or not a > specific name will or won't match an entry returned by readdir. In > those circumstances, even if you think you have cached the entire > directory, you are not guaranteed to know whether the lookup will fail > or succeed. > > -- > Trond Myklebust > Linux NFS client maintainer, Hammerspace > trond.myklebust@xxxxxxxxxxxxxxx > >
