Instances of clurmtabd keep entries relevant to a given mount point in sync cluster-wide. It does this by polling /var/lib/nfs/rmtab periodically.
A single instance of clurmtabd is spawned by the service manager (clusvcmgrd(8)) for each mount point in a given user service. Each instance polls /var/lib/nfs/rmtab every few seconds, looking for changes. When changes are detected, they are merged in with the current in-memory view. The entries are filtered according to the mount point being monitored and are subsequently written out synchronously to that mount point.
When a user service is relocated (or after a fail-over), clurmtabd reads the entries on a given mount point and synchronizes them with the new member's /var/lib/nfs/rmtab. This is done prior to running the exportfs(8) command so that clients of the given service do not receive "Stale NFS file handle" errors.
Clurmtabd is not a part of NFS and can not ensure that "Stale NFS file handle" errors are never received. It is a best-effort attempt. To fully prevent "Stale NFS file handle" errors, it is necessary to modify the Linux kernel as well as nfs-utils.