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
