If you are looking for simple, the easiest way is to have the iSeries tune the system for you. This can be done by setting system value QPFRADJ. For many shops this is sufficient. Auto tuning does a pretty good job if your workloads are fairly consistent, but it doesn't do as well if your workloads vary a lot throughout the day.
In performance tuning, you first have to decide what problem you are trying to solve(interactive performance, batch throughput, etc.) and then find the bottlenecks. For instance, if interactive performance is your issue, check the faulting rates in the various pools by using the WRKSYSSTS command. A consistently high number of non-db faults suggest that your problem might be memory related. An insufficient number of disk arms can also be a performance bottleneck.
I've seen many systems that looked to be maxed out but by just adding disk drives or memory; their useful life was extended by years. Also, check your activity levels by using the WRKSYSSTS command. If the activity levels are set too low and jobs are going to an Ineligible status that will also appear to kill your performance.
I almost forgot the most basic thing to check. Do a WRKACTJOB and sort your jobs on CPU percentage. I've seen many times where a rogue job (programmers doing interactive compiles, people interactively building indexes over large files, etc.) creates problems. If your performance problems seem to pop up at the same time one of these rogue jobs appear, then there's a good chance that may be part of your problem.
The newer iSeries servers have a governor that limits the amount of the CPU that can be used for interactive processing. Once you go over that threshold, your system performance goes downhill very quickly, so try to manage your workloads so that you don't exceed that limit.
This is by no means a complete list of things to do on performance tuning but these are the simple things that I always check first.
Before spending any money for additional hardware, I would suggest running performance tools and making your decisions based on the reports.
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