Capacity planning

Capacity planning turns a measured ceiling into a decision about money and risk. It starts where load-testing finishes: that tells you where the system breaks; this decides how far from that point you choose to run, and what it costs.

Two symmetrical failures. Under-provisioned and you fail during the event you knew was coming. Over-provisioned and you pay for idle capacity indefinitely, which is invisible and therefore never fixed.

1. Know your current ceiling and your current load

You cannot plan without both:

• Ceiling: the point where latency degrades, from a load test. Not the point where it crashes, which is well past useful
• Current peak: actual traffic at its highest, not the average. Averages hide the peak that matters
• Headroom: the ratio between them. This is the number the whole exercise is about