Even though capacity planners naturally tend to focus on the peak minute of the peak hour of the peak day, remember that most complex commercial systems have other stresses in their lives. There are the additional things to factor in. For example:
- Off-peak downtimes where some fraction of the hardware is offline and the total load is carried by what remains
- Nightly reports and other batch jobs
- Activity that is calendar driven like any special end of week, month, quarter, year work that has to be done
- How the available computing power is degraded during site upgrades
- What happens when a part of your computing world briefly goes offline and then the backed up work surges in like a tsunami
Man vs. Machine
Above you see the classic performance signature of machine driven work, a nearly instant-on load with no normal change in intensity as the day progresses. Also, the work runs through the system at a relentless pace because there is no think time.
Sometimes a well thought out solution that works during a peak day kills you at night, kills you on the weekend, or kills you at the end of the month or quarter. Don’t just meter the peaks. Meter all year, and notice when these timed or special events happen. The boss may decide to include these special events in your capacity planning, or they may just decide to take the chance that any special event will happen during a conveniently low demand time. Different businesses are willing to make different tradeoffs between money spent and risks mitigated.
The Human Response
When you present your capacity plan be prepared for at least one round of adjustments. Even with the safety margin and max utilization values built into the calculations, some managers will still be uncomfortable with your results.
Sometimes you are too close to a limit, and the boss will have you add resources to the plan. Sometimes a resource is seen as “too idle”, and that will bother them. Sometimes your results do not support their empire building goals or cost savings targets, and you’ll be asked to change the plan.
Adjustments can be made, but do what you can to make sure the plan sticks to the truth. If your boss tries to force you to lie or profoundly fudge the numbers or “reframe the truth”, do your best to resist.