Comparison
Burst capacityvsHeadroom
Burst capacity
the instance was fast for twenty minutes and then abruptly slow, because it had been spending credits it had run out of.
Capacity available above a baseline for limited periods, either as a cloud instance's credit mechanism or as an autoscaler's fast-path. It is genuinely economical for spiky, low-average workloads and actively dangerous for anything sustained, because performance falls off a cliff rather than degrading. The signature symptom is a service that is fine in testing and slow after an hour in production.
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.
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