Kubernetes Quality of Service (QoS) Classes¶

🧠 What is QoS in Kubernetes?¶

QoS (Quality of Service) defines how Kubernetes prioritizes pods for CPU/memory allocation and eviction under resource pressure.

Every pod falls into one of three QoS classes:

🧩 How Kubernetes Assigns QoS Class¶

QoS is determined per pod, based on the resources (CPU/Memory) defined in each container.

🟢 1. Guaranteed (Highest)¶

Conditions:

• Every container in the pod must define both requests and limits
• For each container, the values of requests must equal limits

resources:
  requests:
    cpu: "1"
    memory: "256Mi"
  limits:
    cpu: "1"
    memory: "256Mi"

📦 Pod QoS Class: Guaranteed

🟡 2. Burstable (Medium)¶

Conditions:

• At least one container sets a resource request or limit
• But not all match exactly, or only requests are set

resources:
  requests:
    cpu: "500m"

📦 Pod QoS Class: Burstable

🔴 3. BestEffort (Lowest)¶

Conditions:

• No requests or limits are defined in any container

resources: {}

📦 Pod QoS Class: BestEffort

⚙️ Why QoS Class Matters¶

🚨 Under Node Memory Pressure (Eviction):¶

Kubelet evicts pods in this order:

BestEffort > Burstable > Guaranteed

🔧 Scheduler decisions (indirectly):¶

While scheduling isn't based on QoS, requests impact scheduling. BestEffort pods don't reserve CPU/memory → easier to schedule but easily evicted.

💥 Examples and Their QoS Classes¶

🧪 How to Check QoS Class of a Pod¶

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

Example:

kubectl get pod myapp -o jsonpath='{.status.qosClass}'

🔍 LimitRange and QoS Class¶

If you do not define resources: explicitly, but a LimitRange exists, Kubernetes auto-applies default requests/limits → your pod becomes Burstable.

Example:¶

apiVersion: v1
kind: LimitRange
metadata:
  name: default-limits
  namespace: dev
spec:
  limits:
  - default:
      cpu: 1
      memory: 512Mi
    defaultRequest:
      cpu: 500m
      memory: 256Mi
    type: Container

Now even if your Pod YAML has no resources, it’ll be assigned Burstable due to these defaults.

🚧 ResourceQuota + LimitRange + QoS: Interaction¶

• ResourceQuota counts requests and limits toward usage.
• If you don't specify anything, LimitRange might assign defaults, consuming your quota.
• This may cause your pod to be rejected unexpectedly if quota is exceeded.

⚠️ Troubleshooting Unexpected QoS¶

❓ Why is my pod not Guaranteed?¶

• Check if every container defines both requests and limits
• Ensure values are equal

❓ Why was my pod evicted first?¶

• Check kubectl describe pod for:

• Evicted

• Status: Failed
• Reason: Evicted
• Message: The node had condition: [MemoryPressure]

📌 Summary: When to Use What¶