To address the issues of CAS, Karpenter uses a different approach. Karpenter directly interacts with the EC2 Fleet API to manage EC2 instances, bypassing the need for autoscaling groups.

The problem occurs when you want to move the pod to another node, in cases such as cluster rebalancing, spot interruptions, and other events. This is because the EBS volumes are zonal bound and can only be attached to EC2 instances within the zone they were originally provisioned in.This is a key limitation that CAS is not able to take into an account when provisioning a new node.

Since Karpenter can schedule nodes quicker, it will most often win this race and provide a new node for the pending workload. CAS will still attempt to create a new node, however will be slower and will most likely have to remove the node after some time, due to emptiness. This brings unnecessary costs to your cloud bill

It’s worth mentioning that Cluster Autoscaler and Karpenter can co-exist within the same cluster.