Problems with Ubuntu 18.04 NFS Clients

We are running a couple of file servers on ReadyNAS 102 devices and recently upgraded our NFS client machines from Ubuntu 16.04 to Ubuntu 18.08.

 

It seems that Ubuntu 16.04 runs NFS version 4.0 and I had selected "Enable NFSv4" on the ReadyNAS boxes. Everything worked fine with our client mounts, which were initiated by the autofs utility.

 

When I switched to 18.04 everything seemed to work fine. However, when the ReadyNAS became more loaded with client connections, we encountered a problem where the NFS client could only mount a single time. The following times it would hang. I could restart the NFS service on the ReadyNAS and the client would then re-attach, but have the same problem the next time it attached. We would also have the problem that new connections would not form on any new clients (that is, rebooting the clients did not help) after a few clients had formed a connection.

 

In diagnosing the problem, we discovered that the Ubuntu 18.04 clients were defaulting to NFS version 4.2 (whereas Ubuntu 16.04 ran NFS version 4.0). We found that when we set the NFS mount flag to explicitly specify a version 4.0 mount, then we could reliably mount, unmount, and remount the ReadyNAS. The nfs mount flag is "-vers=4.0". Since we are using autofs to control the mounts that meant adding a line like the following to our autofs configuration file:

 

1 -vers=4.0 fs1:/home

 

Hope this helps someone else out there. It was very frustrating to figure out.