Quality of Service (QoS) Classes

Overview

Kubernetes assigns QoS classes to pods based on resource requests/limits. QoS determines eviction priority under node pressure.

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QoS Classes

Class	Condition	Eviction Priority
Guaranteed	All containers have requests = limits for CPU & memory	Last (highest priority)
Burstable	At least one container has request < limit	Middle
BestEffort	No requests or limits set	First (lowest priority)

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Guaranteed QoS

Every container must have identical requests and limits:

apiVersion: v1
kind: Pod
metadata:
  name: guaranteed-pod
spec:
  containers:
  - name: app
    image: nginx
    resources:
      requests:
        cpu: "100m"
        memory: "128Mi"
      limits:
        cpu: "100m"       # Must equal request
        memory: "128Mi"   # Must equal request

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Burstable QoS

At least one container has resource requests set, but not equal to limits:

apiVersion: v1
kind: Pod
metadata:
  name: burstable-pod
spec:
  containers:
  - name: app
    image: nginx
    resources:
      requests:
        cpu: "100m"
        memory: "128Mi"
      limits:
        cpu: "500m"       # Different from request
        memory: "256Mi"   # Different from request

---

BestEffort QoS

No resource requests or limits:

apiVersion: v1
kind: Pod
metadata:
  name: besteffort-pod
spec:
  containers:
  - name: app
    image: nginx
    # No resources specified

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Check QoS Class

kubectl get pod <pod-name> -o jsonpath='{.status.qosClass}'

# Or describe
kubectl describe pod <pod-name> | grep "QoS Class"

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Eviction Order

When node is under memory pressure:

1. BestEffort pods evicted first
2. Burstable pods evicted next (based on usage vs request)
3. Guaranteed pods evicted last

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CKA Tip

To get Guaranteed QoS:

• Set both requests AND limits
• Make them equal for BOTH cpu AND memory
• Do this for ALL containers in the pod