I found this on serverfault. It provides usefull internal information about nfs and rpc:

 

* RPC debugging:
       cat /proc/sys/sunrpc/rpc_debug
       echo 2048 > /proc/sys/sunrpc/rpc_debug
       grep . /proc/net/rpc/*/content
       ls -l /proc/fs/nfsd
       cat /proc/fs/nfs/exports 
* NFS debugging:
      # turn on linux nfs debug
      echo 1 > /proc/sys/sunrpc/nfs_debug
      # turn off linux nfs debug
      echo 0 > /proc/sys/sunrpc/nfs_debug