At the moment, three QoS classes are available. Each Pod can have the Guaranteed, the Burstable, or the BestEffort QoS.

Guaranteed QoS

Guaranteed QoS is assigned only to Pods which have set both CPU requests and limits, and memory requests and limits for all of their containers. The Pods we created with the last definition match that criteria.

However, there’s one more necessary condition that must be met. The requests and limits values must be the same per container. There is one more catch. When a container specifies only limits, requests are automatically set to the same values. In other words, containers without requests will have Guaranteed QoS if their limits are defined.

We can summarize criteria for Guaranteed QoS as follows.

Both memory and CPU limits must be set.

Memory and CPU requests must be set to the same values as the limits, or they can be left empty, in which case they default to the limits (we’ll explore them soon).

Pods with Guaranteed QoS assigned are the top priority and will never be killed unless they exceed their limits or are unhealthy. They are the last to go when things go wrong. As long as their resource usage is within limits, Kubernetes will always choose to kill Pods with other QoS assignments when resource usage is over the capacity.

Let’s move to the next QoS.