Rebuild a Kubernetes Node Without Downtime

When I built the in-house Kubernetes cluster with Raspberry PIs, I followed the kubeadm instructions and installed Raspberry PI OS on the PIs. It was all good except the RPI OS is 32-bit. Now I want to install a Ubuntu 20.04 Server ARM64 on this PI, below are steps with which I rebuilt the node with Ubuntu and without disrupting the workloads running in my cluster.

First, I didn’t need to shutdown the running node because I’ve got a spare MicroSD card to prepare the Ubuntu image. The instruction for writing the image to the MicroSD card is here. When the card is prepared by the Imager, I kept it in the card reader because I wanted to set the IP address instead of the automatic IP by default. A fixed IP makes more sense if I want to connect to it, right?

To set a static IP in the Ubuntu MicroSD card, open system-boot/network-config file with a text editor and put in something like this:

version: 2
ethernets:
  eth0:
    # Rename the built-in ethernet device to "eth0"
    match:
      driver: bcmgenet smsc95xx lan78xx
    set-name: eth0
    addresses: [192.168.1.82/24]
    gateway4: 192.168.1.1
    nameservers:
      addresses: [192.168.1.1]
    optional: true

Now the new OS is ready. To gracefully shutdown the node, drain it with

kubectl drain node-name
# wait until it finishes
# the pods on this node will be evicted and re-deployed into other nodes
kubectl delete node node-name

Then I powered down the PI and replaced the MicroSD card with the one I just prepared, then I powered it back on. After a minute or 2, I was able to ssh into the node with

# wipe the previous trusted server signature
ssh-keygen -R 192.168.1.82
# login, default password is ubuntu and will be changed upon first login
ssh [email protected]
# install ssh key, login with updated password
ssh-copy-id [email protected]

The node needs to be prepared for kubeadm, I used my good old ansible playbook for this task. The ansible-playbook command looks like

ansible-playbook -i inventory/cluster -l node-name kubeadm.yaml

At the moment I have to install less recent versions of docker and kubeadm to keep it compatible with the existing cluster.

When running kubeadm join command I encountered an error message saying CGROUPS_MEMORY: missing. This can be fixed with this. And one more thing is to create a new token from the master node with command:

kubeadm token create

At last the new node can be joined into the cluster with command:

kubeadm join 192.168.1.80:6443 --token xxx     --discovery-token-ca-cert-hash sha256:xxx

The node will then be bootstrapped in a few minutes. I can tell it’s now ARM64

k get node node-name -o yaml |rg arch
    beta.kubernetes.io/arch: arm64
    kubernetes.io/arch: arm64
...

🙂

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