MIME-Version: 1.0
From:   James Pearson <jcpearson@gmail.com>
Date:   Wed, 20 Feb 2019 11:28:53 +0000
Message-ID: <CAK3fRr9EL-YOp3yUr5tMe5zuiERxHjkLDhAaWkxxEHjtfSwisQ@mail.gmail.com>
Subject: nfsd thread limit and UDP ?
To:     linux-nfs@vger.kernel.org
Content-Type: text/plain; charset="UTF-8"
Sender: linux-nfs-owner@vger.kernel.org
Precedence: bulk

On a very busy NFSv3 server (running CentOS 6), we recently upped the
nfsd thread count to 1024 - but this caused client mount requests over
UDP to fail.

We configure all our clients to use TCP for NFS mounts, but the
automounter (automountd) on MacOS (up to version MacOS 10.12) seeds a
'null call' to the NFS server over UDP before attempting the mount -
but the server appears to ignore any UDP requests - and the automount
fails

I can also reproduce the issue on a Linux client via:

 mount -o udp,nfsvers=3 server:/export /mount/point

I've found, by trial and error, that the maximum number of nfsd
threads that can be run on the server is 1017 before UDP mount
requests fail

Running tcpdump on the server shows the UDP requests from the client,
but the server never replies

It looks like more recent versions of MacOS will do its test 'null
call' over TCP - so that is one 'solution' to this issue

However, I'm interested to know if we're hitting some hard limit, or
if there are any settings we can tweak that could mitigate the
problem?

Thanks

James Pearson