High latency events in event log

Hello,

Could someone tell me a about event log meesages that have the form:

"Storage domain X experienced a high latency of X from host X..."

We are recieving them regularly now for all of our storage domains.  We have not noticed a slight drop off in VM performance over the last year or so - nothing significant however.  RHEV is version 3.0, NFS data center.

What sort of test is done to measure latency?  Thanks!

DW