# QoS Classes and Eviction — Kubernetes

Source: https://www.skillbyai.com/en/kubernetes/r-qos

> Who gets evicted first under pressure.

## Guaranteed, Burstable, BestEffort

Kubernetes assigns each pod a **Quality of Service** class from its resources: **Guaranteed** (every container has requests equal to limits for CPU and memory), **Burstable** (some requests set), **BestEffort** (none set). When a node runs low on memory, the kubelet **evicts** pods, BestEffort first, then Burstable pods using the most above their requests. Critical workloads should be Guaranteed or at least have accurate requests; **PriorityClasses** further decide which pods may preempt others.

## Resources for each QoS class

Not applied to a live cluster in this course; check field names against the API reference for your version.

```yaml
# Guaranteed: requests == limits for cpu and memory in every container
resources: {requests: {cpu: "500m", memory: 512Mi}, limits: {cpu: "500m", memory: 512Mi}}

# Burstable: requests set, limits higher (or missing)
resources: {requests: {cpu: "250m", memory: 256Mi}, limits: {memory: 512Mi}}

# BestEffort: no requests or limits at all (first to be evicted)
resources: {}
```

## Never run production pods as BestEffort

Pods without requests are the first to be evicted and can starve neighbours.

**Quiz:** Which pods are evicted first when a node runs out of memory?

- [ ] The newest pods regardless of class
- [ ] Guaranteed pods
- [x] BestEffort pods with no requests or limits
- [ ] Pods with the longest names

*Answer:* BestEffort pods with no requests or limits. Set requests to protect important pods.