How to identify which NFS client generates disk IO?

iotop and then o - you will see which process reads and/or writes and how much to the HDD.

Check the pid of that process and do netstat -entp | grep <pid> - that way you will see established tcp connection and from which address it's coming. Use enp to check for both tcp and udp sessions.

You can also do a netstat -anp | grep 2049 - that way getting an ip address and pid, then correlate the pid to the one from iotop.

Usually the client using most IO will also doing most network traffic, so what I do is: dump all traffic for a few seconds, and then create a sorted list of the hosts (limited to the nfs hosts) that used most traffic:

tcpdump > dump.cap  # (30 secs should be enought), press ctr+ c
grep -o "<something iding an nfs client>" dump.cap | sort | uniq -c | sort -n