Note: this only applies to devstack running with Ubuntu 16.04 as the host system.
If tests begin failing with an nfs error (something related to 'no locks available'), for example:
OSError: [Errno 37] No locks available
It is due to the statsd service not being started. NFS relies on statsd for providing locks.
Run the following commands on your host OS (>= Ubuntu 16.04)
sudo systemctl enable rpc-statd # Enable statd on boot
sudo systemctl start rpc-statd # Start statd for the current session
You shouldn't need to reboot or reload Vagrant but if the problem persists and you continue to get errors related to 'no locks available' a 'vagrant reload' should do the trick.