Autoscaler does not scale up nodes after updating min-size annotation

Products

• VMware vSphere Kubernetes Service

Issue/Introduction

• After updating the Autoscaler annotation to increase the node pool minimum size from x to y, the nodes in the vSphere Kubernetes cluster did not scale up as expected. 
• The annotation in the MachineSet was changed from : 
  

  cluster.x-k8s.io/cluster-api-autoscaler-node-group-min-size: "x"

  to: 
  

  cluster.x-k8s.io/cluster-api-autoscaler-node-group-min-size: "y"

   

• Despite this modification, the cluster did not automatically add new nodes. 

Environment

VMware vSphere Kubernetes Service

Cause

This behavior is expected. The Cluster Autoscaler does not scale nodes based solely on annotation changes. The min-size annotation defines the boundary i.e. minimum allowed nodes, not a trigger to immediately scale up.

Autoscaler only provisions additional nodes when:

• Pods cannot be scheduled due to lack of resources.
• Existing nodes cannot accommodate pending workloads.

Resolution

No action is required unless there is an actual resource shortage in the cluster.

The Autoscaler will: 

1. Ensure the cluster never falls below the defined min-size (in the above example - y).
2. Scale up only when pods are pending and require additional compute.
3. Scale down if nodes become unnecessary, but never below the min-size boundary.

Changing the annotation alone will not cause immediate scale-up

Additional Information

About Cluster Autoscaling