Safety and Suitability

Why It’s Safe for a Home Lab

1. Simplicity:

   • A single master node reduces complexity compared to a high-availability (HA) setup with multiple masters. For a home lab focused on experimentation, learning, or running non-critical applications (e.g., Nextcloud, Vaultwarden, Paperless-ngx, Bookstack), this simplicity is a major advantage.

   • k3s is designed to be lightweight and can run the control plane and workloads on a single node if needed, making it forgiving for small setups.

2. Resource Efficiency:

   • With only four VMs (each with 2 vCPUs and 4GB RAM), dedicating one VM as a master and three as workers optimizes resource usage. Adding more masters would require additional VMs or higher specs, which might strain your two physical machines.

3. k3s Design:

   • Unlike full Kubernetes, k3s uses an embedded SQLite database by default for the control plane (instead of etcd), which simplifies single-node operation. This reduces the risk of database-related failures that might plague a traditional single-master Kubernetes setup.

4. Worker Redundancy:

   • Three worker nodes provide decent resilience for your application pods. If one worker fails (e.g., VM3 on Physical Machine 2), the remaining two (VM2 and VM4) can still run your workloads, assuming proper pod scheduling and replicas.

5. Home Lab Context:

   • For personal use, occasional downtime (e.g., during maintenance or a failure) is typically acceptable. The single-master setup is "safe" in that it meets the needs of a non-production environment without overcomplicating things.

Risks of a Single Master Node

1. Single Point of Failure (SPOF):

   • The master node (VM1 on Physical Machine 1) hosts the Kubernetes control plane (API server, scheduler, controller manager). If VM1 or Physical Machine 1 fails, you lose the ability to manage the cluster:

     • You can’t deploy new pods, scale applications, or update configurations until the master is restored.

     • Existing pods on worker nodes (VM2, VM3, VM4) will continue running as long as their nodes are operational, but they won’t reschedule or recover from failures without the control plane.

2. Physical Machine Dependency:

   • Since VM1 (master) is on Physical Machine 1, a failure of that physical machine (e.g., power loss, hardware issue) takes down the master and VM2 (a worker), leaving only VM3 and VM4 operational. This reduces your cluster’s capacity significantly.

3. Limited High Availability:

   • With one master, you don’t get the fault tolerance of an HA setup (e.g., three masters with etcd). Any disruption to the master requires manual intervention to restore it.

4. Storage Risks:

   • If the master’s storage (e.g., where /var/lib/rancher/k3s is stored) becomes corrupted or fails, you could lose cluster state, requiring a rebuild unless you have backups.

Mitigations for Safety

To make this 1-master, 3-worker setup safer and more reliable, consider these steps:

1. Backups:

   • Regularly back up the master node’s k3s directory:

     bash

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     tar -czf k3s-backup.tar.gz /var/lib/rancher/k3s

   • Store backups on an external drive or Physical Machine 2. This allows you to restore the cluster state on a new VM if VM1 fails.

2. Worker Node Resilience:

   • Configure your deployments with multiple replicas (e.g., replicas: 2 or 3) and anti-affinity rules to spread pods across VM2, VM3, and VM4. This ensures that a single worker failure doesn’t disrupt your applications:

     yaml

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     apiVersion: apps/v1
     kind: Deployment
     metadata:
      name: nextcloud
     spec:
      replicas: 2
      selector:
        matchLabels:
          app: nextcloud
      template:
        metadata:
          labels:
            app: nextcloud
        spec:
          affinity:
            podAntiAffinity:
              preferredDuringSchedulingIgnoredDuringExecution:
              - weight: 100
                podAffinityTerm:
                  labelSelector:
                    matchExpressions:
                    - key: app
                      operator: In
                      values:
                      - nextcloud
                  topologyKey: kubernetes.io/hostname
          containers:
          - name: nextcloud
            image: nextcloud:latest

3. Monitor the Master:

   • Set up basic monitoring (e.g., with a tool like Prometheus or a simple script) to alert you if VM1 becomes unresponsive. This lets you act quickly if there’s an issue.

4. Quick Recovery Plan:

   • Document the steps to spin up a new master VM on Physical Machine 2 (e.g., VM5) and restore from backup. Rejoin the workers using the restored token and IP:

     bash

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     curl -sfL https://get.k3s.io | K3S_URL=https://<new-master-ip>:6443 K3S_TOKEN=<restored-token> sh -

5. Spread Critical Data:

   • Use NFS storage across both physical machines (as in your setup) to ensure data availability even if Physical Machine 1 fails. This protects your app data (e.g., Nextcloud files, Vaultwarden vault).

6. Test Failure Scenarios:

   • Simulate a master failure (e.g., shut down VM1) to confirm that your worker nodes keep running existing pods and that you can recover the cluster. This builds confidence in the setup’s safety.

When Is It Not Safe?

A single master node might not be ideal if:

• Uptime Is Critical: If you rely on these apps daily (e.g., Nextcloud for work files), downtime during a master failure could be disruptive.

• Heavy Workloads: If your cluster grows significantly (more apps, higher traffic), a single master might become a bottleneck or fail under load.

• No Backup Strategy: Without backups, losing the master could mean rebuilding the cluster from scratch, losing all configurations.

For these cases, an HA setup with 3 masters (requiring at least 3 VMs for the control plane plus workers) would be safer, but it’s overkill for most home labs given the added resource and complexity costs.

Conclusion

Yes, a 1-master, 3-worker k3s cluster is safe enough for a home lab focused on learning, personal projects, or non-critical services. The risks (primarily the SPOF of the master) are manageable with backups, worker redundancy, and a recovery plan. Your setup leverages three workers for workload resilience and keeps resource demands low, aligning well with a home lab’s goals.

If you’re comfortable with occasional manual intervention and have backups in place, this configuration is practical and secure for your needs. For extra peace of mind, implement the mitigations above, and you’ll have a robust, low-maintenance cluster tailored to your environment!