Step-by-Step Guide: In-Place Vertical Scaling for Pod-Level Resources in Kubernetes v1.36

Introduction

Kubernetes v1.36 brings a powerful enhancement: the In-Place Pod-Level Resources Vertical Scaling feature has graduated to Beta, enabled by default via the InPlacePodLevelResourcesVerticalScaling feature gate. This allows you to update the aggregate pod resource budget (.spec.resources) for a running pod without always restarting containers. This guide walks you through the process step by step, from prerequisites to verification.

What You Need

• A Kubernetes cluster running v1.36 or later (the feature gate is enabled by default).
• kubectl configured to communicate with your cluster.
• Basic understanding of pod-level resources and cgroups.
• A running pod that uses pod-level resource limits (without individual container limits) to benefit from in-place resizing.

Step-by-Step Instructions

Step 1: Verify Cluster Version and Feature Gate

Confirm your cluster is on v1.36+ and the feature is active. Run:

kubectl version --short

Check that the InPlacePodLevelResourcesVerticalScaling feature gate is enabled (it is by default). Ensure your nodes support container runtime interface (CRI) in-place updates (most modern runtimes do).

Step 2: Define a Pod with Pod-Level Resources

Create a YAML file like shared-pool-app.yaml. The pod must define spec.resources (the pod-level limit) and omit individual container limits so containers inherit the shared pool. Example:

apiVersion: v1
kind: Pod
metadata:
  name: shared-pool-app
spec:
  resources:
    limits:
      cpu: "2"
      memory: "4Gi"
  containers:
  - name: main-app
    image: my-app:v1
    resources: {}
    resizePolicy:
    - resourceName: "cpu"
      restartPolicy: "NotRequired"
  - name: sidecar
    image: logger:v1
    resources: {}
    resizePolicy:
    - resourceName: "cpu"
      restartPolicy: "NotRequired"

Note: The resizePolicy is set per container. Currently, pod-level resizePolicy is not supported—the Kubelet defers to individual container settings.

Step 3: Apply the Pod

Deploy the pod:

kubectl apply -f shared-pool-app.yaml

Wait for the pod to become Running. Verify it has no individual container limits:

kubectl describe pod shared-pool-app | grep -A5 Limits

You should see only the pod-level limits listed under Status or Spec.

Step 4: Initiate a Resize Operation

To double the CPU pool from 2 to 4 CPUs, use the resize subresource. This is the recommended way to perform in-place vertical scaling:

kubectl patch pod shared-pool-app --subresource resize --patch '{"spec":{"resources":{"limits":{"cpu":"4"}}}}'

This triggers the Kubelet to adjust the cgroup limits. The resizePolicy determines whether containers restart: NotRequired (non-disruptive) updates cgroups dynamically via CRI; RestartContainer restarts the container.

Step 5: Verify the Resize

Check the pod status for a ResizedPod condition or view the new limits:

kubectl describe pod shared-pool-app | grep -i -A2 "Pod Resources"
kubectl get pod shared-pool-app -o jsonpath='{.status.podResources}'

If containers were not restarted, you should see the new CPU limit (4) applied instantly. Use kubectl top pod shared-pool-app to monitor resource usage.

Step 6: Understand ResizePolicy Behavior

The Kubelet checks each container's resizePolicy:

• NotRequired: The Kubelet attempts a non-disruptive cgroup update via CRI. If the runtime supports it, no restart occurs.
• RestartContainer: The container is restarted to apply the new pod-level boundary safely.

If a container has no resizePolicy defined, the Kubelet defaults to NotRequired.

Step 7: Monitor Node Stability and Kubelet Checks

When you apply a resize patch, the Kubelet performs a sequence of checks before applying changes:

1. Feasibility: Verifies the requested resources don't exceed node capacity.
2. Safety: Ensures the new pod-level limits are compatible with existing containers (e.g., no negative values).
3. Inheritance: For containers without individual limits, recalculates effective boundaries from the new pod-level pool.

If any check fails, the resize is rejected and the pod remains unchanged. Monitor events:

kubectl get events --field-selector involvedObject.name=shared-pool-app

Tips for Success

• Start with non-disruptive policies: Use NotRequired for critical containers that must stay alive during resizing.
• Use resize subresource: Always use --subresource resize rather than directly patching spec.resources to ensure the Kubelet handles the update correctly.
• Test in a non-production environment: Verify that your container runtime supports in-place cgroup updates (most do, including containerd and CRI-O).
• Combine with Horizontal Pod Autoscaler: In-place vertical scaling complements HPA by allowing you to adjust pod-level budgets when individual container limits are not defined.
• Watch for sidecar containers: If sidecars have strict resource requirements, define individual limits to avoid unexpected sharing.
• No pod-level resizePolicy yet: Currently, you must set resizePolicy on each container. A future release may introduce a pod-level default.

With this guide, you can effectively leverage in-place vertical scaling for pod-level resources in Kubernetes v1.36, reducing downtime and simplifying resource management.