Linux NFS Server
To monitor a Linux NFS server, an extension to the snmpd.conf is required
Add this line:
extend lm-nfsd /bin/cat /proc/net/rpc/nfsd
to snmpd.conf, and restart the snmpd daemon.
The Linux NFS Server stats- datasource will then discover and monitor the performance of the NFS server. (You can manually trigger ActiveDiscovery to speed up that process.)
NFS mounted filesystems
The NET-SNMP binaries do not deal with terabyte file systems well if they are NFS mounted - they will have small hrStorageAllocationUnits, and hrStorageSize and hrStorageUsed will wrap, so you may see that the Used space is greater than the total Size. There is currently no fix for this in the NET-SNMP SNMP collector, so the workaround is to monitor the NFS server, not the system that mounts them.