Migrating to containerd

Since dockershim has been deprecated and soon (as of kubernetes 1.22+) to be removed, there is a need to migrate cluster container-runtime to some other CRI compatible container runtime. At the moment KubeOne supports only containerd as a second option.

The cluster is divided into 3 groups of nodes:

• control-plane nodes
• static workers
• dynamic workers

and each one needs to be migrated.

Before migration

Config

All kubernetes versions prior 1.22+ (where switch will happen) have to explicitly specify the containerd as a container runtime in the config. For this please use containerRuntime.containerd field as shown below.

apiVersion: kubeone.io/v1beta1
kind: KubeOneCluster

versions:
  kubernetes: "1.21.1"

containerRuntime:
  containerd: {}

Migrate control-plane and static-workers

Once the config is set to containerd:

terraform output -json > tf.json
kubeone migrate to-containerd -t tf.json

This step will do the following on each control-plane and static-worker node:

• reconfigure kubelet to communicate over CRI to containerd
• reconfigure containerd to enable CRI plugin
• stop ALL docker containers
• remove ALL docker containers
• restart kubelet
• wait (with 10 minutes timeout) for all containers to became ready on give node

Migrate dynamic-workers

Once previous step is completed, machine-controller deployment will be reconfigured with -container-runtime=containerd flag. But your existing dynamic-workers still using docker. In order to get rid of docker in dynamic-workers you need to rolling recreate dynamic-workers. To do that please force machine-controller to recreate Machines. This can be done by patching MachineDeployment objects at spec.template.metadata.annotations for example.

See more at Rolling Restart MachineDeploments

Outro

If you wish to, it’s now safe to uninstall docker / docker-cli. Dockerd is still running in your control-plane and static-workers nodes, it’s just kubelet now configured to not use it anymore.