Comparison
Capacity planningvsHeadroom
Capacity planning
you work out from load tests how much traffic one instance handles, and how many you need for Black Friday.
Determining the resources required to meet demand at a target latency, with headroom for failure and spikes. It requires a measured per-instance capacity, not an assumed one. Plan for the loss of a zone or a region as well as the peak, or your headroom disappears exactly when you need it.
Full entry →Headroom
you run at sixty percent so a zone failure or a traffic spike does not immediately become an outage.
Deliberate unused capacity held to absorb failures, spikes and the time it takes to add more. The right amount is set by what you must survive without scaling — commonly the loss of one zone — plus however long provisioning actually takes. It is the least glamorous line on a cloud bill and the first thing cut in a cost exercise, usually by someone who has not connected it to the last incident.
Full entry →