Performance problems are often reported immediately following some change to system hardware or software. Unless there is a pre-change baseline measurement with which to compare post-change performance, quantification of the problem is impossible.
Having the Performance Diagnostic Tool (PDT) installed and operational (see Chapter 13. Using Performance Diagnostic Tool (PDT)) provides a baseline of overall system performance.
Changes to any of the following can affect performance:
One option is to run the measurement before and after each change. The alternative is running the measurements at regular intervals (for example, once a month) and save the output. When a problem is found, the previous capture can be used for comparison. It is worth collecting a series of outputs in order to support the diagnosis of a possible performance problem.
To maximize performance diagnosis, collect data for various periods of the working day, week, or month when performance is likely to be an issue. For example, you might have workload peaks as follows:
Use measurements to collect data for each of these peaks in workload, because a performance problem might only cause problems during one of these periods and not during other times.
Note: Any measurement has an impact on the performance of the system being measured.