What I want to know is from the output of top given above, can we be sure that there is an I/O issue on the server ?All you can say is that during a certain 1 minute time interval in the past, 15 CPU's were theoretically needed to have no process in the waiting queue. However, it does not tell you anything about the cause. For instance, what if the problem was due to network congestion or routing issue outside your computer? The problem with any such statistics is that they cannot take your expectation of efficiency for a given task or time frame into the consideration. The results may even be even be very good for what the system was doing at the time.