Re: [bug report] nfs clients fail to get read delegations after file open with O_RDWR

["J. Bruce Fields" <bfields@fieldses.org>]

On Tue, Dec 14, 2021 at 12:19:54PM -0500, Jeff Layton wrote:
> On Tue, 2021-12-14 at 11:45 -0500, J. Bruce Fields wrote:
> > On Tue, Dec 14, 2021 at 12:50:53AM +0000, suy.fnst@fujitsu.com wrote:
> > > If I understand the case correctly, the most common workload it influences like:
> > > 
> > > One nfs client opens a file with flag O_WRONLY/O_RDWR, close it.
> > > Then some nfs clients open the file with O_RDONLY right now then it will prevent
> > > server to give any delegation to other clients.  It may cause many unnecessary
> > > requests from clients because lack of delegations.
...
> Is that really desirable behavior? There is the bloom filter in
> nfs4state.c too that prevents it from handing out a delegation too soon
> after a delegrecall.
> 
> The situation above doesn't involve a recall, but it _could_ have if the
> timing had been a little different. It's probably worth thinking about
> how the rules for this ought to work in all cases.
> 
> Should we be treating inodes that experience real delegation recalls
> differently from this case?

It's not hard to imagine common cases where clients would want to read a
file soon after it's written.  E.g. a distributed compile or processing
pipeline where clients are sitting there waiting for new files to appear
and then doing further processing on them when they do.  I guess the
creator would do something in that case to indicate when the writing was
done--maybe rename the file to a common directory, or send some kind of
signal outside of NFS.

--b.