How the Network Performance Feature records time

While you are analyzing timestamped data in the Network Performance Feature reports, you should remember that the time listed on the report for an event is not the exact time the event occurred. The following figure illustrates how the Network Performance Feature records event times.

Figure 1. Recording time in the Network Performance Feature

The times listed in the figure are:

t0: Time when the event occurred
t1: Time when the NetView collect period starts
t2: Time when the NetView collect period ends
t3: Time when the event is written to SMF

NetView starts writing records to SMF directly after the collect period ends, so t2 is almost equal to t3. The Network Performance Feature stores t3 as the time of the event, rather than t0.

Because of the method the Network Performance Feature uses to record the time of an event, the length and frequency of the NetView collect period affects the accuracy of the event times. For example, if you set the NetView collect period to 5 hours, events occurring at the beginning and the end of that period would be logged in the Network Performance Feature as occurring at the same time.

Figure 2. Example using a long NetView collect period

In the example illustrated, the Network Performance Feature records both events as occurring at just after 09:15, which is the time the data is recorded to SMF. This time is fairly accurate for Event 2, but not for Event 1. If you need to more accurately establish the time for Event 1, you would shorten the collect period and collect data more often.

However, increasing the frequency of collect periods increases the number of SMF records generated and collected by the Network Performance Feature. Because the performance of IBM Z Decision Support and the Network Performance Feature decreases as the number of SMF records collected increases, you must be careful not to shorten the collect interval too much.