Lesson 1: Migration Strategy and Planning
Migrating from k3s to RKE2 / Introduction and Migration Strategy
A successful zero-downtime migration requires careful planning before we touch a single node. This lesson develops a phased strategy for moving from k3s to RKE2 and maps the risks involved at each step.
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This is the 1st lesson of the guide Migrating from k3s to RKE2. The full list of lessons in the guide can be found in the overview.
Migrating a production Kubernetes cluster is one of the most demanding operations in infrastructure management. Our migration must maintain zero downtime while keeping services available, change the underlying distribution from k3s to RKE2, and reconfigure the topology from a single control plane with two workers to three control planes with one worker, all at the same time. We are also replacing the operating system with Rocky Linux 10, upgrading to Canal for networking and setting up Longhorn for storage.
The current k3s cluster has Node 1 as its sole control plane, resulting in a single point of failure that puts the entire cluster at risk if that node goes down.
Storage relies on local volumes per node with no replication, and Flannel provides basic CNI networking with external ingress routed directly to fixed node IPs.
The target RKE2 cluster addresses each of these limitations:
| Aspect | Current (k3s) | Target (RKE2) |
|---|---|---|
| Control Plane | Node 1 only (single point of failure) |
Nodes 2, 3, 4 (HA with etcd quorum) |
| Workers | Nodes 2, 3 |
Node 1 (extensible to more) |
| Storage | Local storage per node | Longhorn replicated + local-path |
| CNI | Flannel | Canal (Flannel + Calico) + Wireguard |
| Ingress | Fixed node IPs | Traefik DaemonSet + Hetzner Cloud LB |
The migration happens in five phases, each moving one step closer to the target architecture while preserving service availability.
This phase creates a new RKE2 cluster on Node 4 while Cluster A remains fully operational with all three nodes.
We install Rocky Linux 10 on Node 4, configure the Hetzner vSwitch networking, and install RKE2 as the first control plane with Canal as the bundled CNI plugin.
After verifying cluster functionality, Node 4 runs as a single-node RKE2 cluster while Nodes 1-3 continue serving workloads unchanged.
Node 3 leaves Cluster A and joins Cluster B as a second control plane, giving the new cluster its first step toward high availability.
Before beginning, we verify that all workloads run on Nodes 1 and 2, DNS does not point to Node 3, and external traffic is routed elsewhere.
The process involves cordoning and draining Node 3, removing it from Cluster A, optionally reinstalling the OS with Rocky Linux 10, and joining it as an RKE2 control plane.
After verifying etcd cluster health, both clusters operate at minimum viable capacity Cluster A with two nodes and Cluster B with two etcd members, neither of which tolerates losing a node.
Node 2 follows the same process: cordon, drain, remove from Cluster A, uninstall k3s, optionally reinstall the OS, and join Cluster B as the third control plane.
With three etcd members, Cluster B achieves full high availability and it can tolerate the loss of one control plane node while maintaining quorum.
Workload migration can now begin safely.
With Cluster B running three control planes and both clusters fully operational, the risk of this phase is low as DNS can be switched back to Cluster A if issues arise.
We set up storage on Cluster B with Longhorn and local-path provisioner, configure ingress through Traefik and the Hetzner Cloud Load Balancer, and export workload manifests from Cluster A.
After migrating any persistent data and deploying workloads to Cluster B, we switch DNS to point at the new cluster’s ingress.
All workloads now run on Cluster B while Cluster A sits idle with only Node 1.
The final phase decommissions Cluster A and brings Node 1 into the RKE2 cluster as a worker node.
After a 24-48 hour soak period to verify Cluster B stability, we drain Node 1, uninstall k3s, optionally reinstall with Rocky Linux 10, and join it as an RKE2 agent.
The result is a complete 4-node RKE2 cluster with three control planes and one dedicated worker.
The highest-risk phase is Phase 2, when both clusters run at minimum viable capacity.
Cluster A loses one of its three nodes, and Cluster B has only two etcd members, which has zero fault tolerance, the same as a single-node cluster.
We minimize time in this state by ensuring both nodes are stable and proceeding to Phase 3 as quickly as practical.
We ideally never proceed to workload migration until Cluster B achieves full HA with three control plane nodes.
Before starting the migration, we review all lessons thoroughly, practice the node installation process on a test system if possible, and ensure we have complete backups of all persistent data.
Lesson Conclusion
Congratulations! Our migration plan defines five phases, each with clear entry and exit criteria, and maps the risk profile from bootstrap through final consolidation. You have completed this lesson and you can now continue with the next one.
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