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In Cleura Cloud, you have the option of putting a Gardener-based Kubernetes cluster in hibernation. This is something you might want to do whenever you know you won’t be needing the cluster for some time. Putting it in hibernation makes sense because from that time on and before you wake it up, you pay less for hosting.

You may wonder why you still pay something after putting a cluster in hibernation, instead of paying nothing at all. To answer this question, we have to explain how hibernation works.

How hibernation works

Learning about the logic of hibernation in Gardener, we immediately get a fresh perspective on the whole concept. When we hear about hibernation, we usually think of resources that are merely stopped or frozen, and their volatile information made persistent. This is not how Gardener hibernation works — and here’s why.

When you put a Gardener cluster in hibernation, what really happens is that all worker nodes are removed. So, as long as the cluster is in hibernation, you will not be paying for CPU, RAM, and boot volume storage utilization incurred by the worker nodes. But there might still be resources created by the cluster, for which you will keep getting charged even when the cluster is in hibernation. More specifically, you will keep paying for any persistent volumes, floating IPs, and load balancers associated with the cluster.

In Kubernetes, a Persistent Volume (or PV), is a piece of storage in the cluster that has been provisioned either dynamically (using Storage Classes) or by an administrator. When a user makes a request for storage, then we have a PersistentVolumeClaim (or PVC). Floating IPs, on the other hand, are instantiated to be assigned to Kubernetes Services that need a public IP. Those services have an external load balancer (i.e., a service of Type: LoadBalancer). When you hibernate a Gardener cluster, objects of any of those types are retained.

Waking up clusters

When you wake a hibernated cluster, the previously removed worker nodes reappear. Since all information about cluster resources (Pods, Deployments, ReplicaSets etc.) are still available on the control plane, these cluster resources also automatically reappear on the recreated worker nodes.

However, to run your Pods, Gardener will have to re-fetch any images they previously ran on. That is because the image cache of newly created worker nodes starts empty. Consequently, please keep in mind that a cluster whose resources were perfectly humming along before the hibernation, might suddenly see Pods failing with ImagePullError if any image they depend on has been deleted in an upstream registry.

Last but not least, the act of waking a cluster up may temporarily fail, because while the cluster was hibernating, the tenant came close to its volume, RAM, CPU, etc. quota, and attempting to re-instantiate the worker nodes and re-activate the cluster would breach the quota limit.

Last update: 2023-03-29
Created: 2023-03-21
Authors: Christos Varelas, Florian Haas