Unmount data volume hosting NFS exports

So I've got a customer with two NFS server setup in a pacemaker cluster active/standby configuration. It is a legacy system running RHEL6. On the servers there is /mnt/data1 which is an xfs mountpoint on a drbd mirrored disk. The mount is active on one node at a time and controlled by pacemaker (so is drbd for that matter). My problem is in critical cases where I need to move active services to the other server without shutting down the nfs clients first. I can shutdown the NFS services, but no matter what I try, I can't unmount the /mnt/data1 filesystem as it reports as 'busy'. I tried changing the daemon stop sequence on the nodes. Right now I have the following sequence: - rpc.mountd - nfsd - exportfs -au - rpc.statd Both 'lsof /mnt/data1' and 'fuser -mv /mnt/data1' do not report any open files on the mountpoint and I can verify no terminal sessions are there either. Short of having to shutdown the box (which kills any debugging I would like to do), I can't get the filesystem unmounted to allow pacemaker to cleanly move the filesystem mount to the other node. I assume that there are some hanging file locks, but I'm not sure how else to kill them. Any ideas are appreciated.