rsync between one nfs nas mountpoint to another nas mountpoint freezes

Issue

• Rsync has been initiated from one nas mountpoint to another nas mountpoint, it continues for a while and then freezes.
• In the meantime noticed CPU wait is almost 100% and the rsync process itself becomes zombie.

# !while
while true; do date; ps auxf | awk '{if($8=="D") print $0;}'; sleep 1; done
Wed Jun  1 13:41:32 NZST 2016
root     31090  0.0  0.0 100988   848 ?        D    13:25   0:00  \_ df -k
root     30259  0.1  0.6 150956 26116 pts/0    D    13:21   0:01  |                           \_ rsync -avu --progress /nas1/ /nas2/

• Test environment - but this needs to be done on several production hosts without impacting the OS and normal function.