K3OS Raspberry Pi 4 Architecture and Deployment

The implementation of container orchestration on single-board computers represents a critical intersection between edge computing and infrastructure automation. While traditional Linux distributions such as Raspberry Pi OS or Ubuntu are common starting points, they often introduce overhead and configuration complexities that can destabilize a Kubernetes environment. K3OS emerges as a specialized solution, functioning as a Linux distribution specifically engineered to host K3s. Unlike general-purpose operating systems, k3OS treats the operating system as a component of the cluster, allowing the entire configuration to be declared via YAML files. This shift toward a declarative model enables the pre-configuration of images before the hardware ever boots, eliminating the manual "hoops" typically associated with networking and DNS setup in standard environments. For the Raspberry Pi 4, this architecture is particularly potent, as it mitigates the systemic instabilities observed in heavier distributions where pods may fail frequently due to resource contention and improper kernel tuning.

Hardware Foundations and Resource Requirements

Running a K3s cluster on Raspberry Pi hardware requires a strict adherence to hardware specifications to avoid the catastrophic failures associated with memory exhaustion. While k3OS is designed for efficiency, the underlying hardware must support the resource demands of the Kubernetes control plane and the subsequent workloads.

• RAM Capacity: A minimum of 4GB of RAM is required for stable operation, although 8GB is strongly recommended for larger workloads to prevent the k3s daemon from failing to start due to insufficient memory.
• Storage Media: High-quality microSD cards (32GB or larger) are the baseline, though SSDs connected via USB are preferred for booting to improve I/O performance.
• Power and Connectivity: A stable power supply and a reliable network connection are mandatory to prevent node flapping and data corruption.

The impact of these hardware choices is most evident in the storage layer. K3s typically utilizes an embedded etcd as the Control Plane Database. However, etcd is known to suffer from performance issues on slower disks, specifically microSD cards common in Raspberry Pi setups. This creates a bottleneck that can lead to cluster instability. To resolve this, the control plane database can be externalized to a more robust environment, such as a PostgreSQL instance running in Docker on a separate machine (e.g., a Mac), thereby removing the heavy I/O load from the Pi's SD card.

K3OS Declarative Configuration and Image Generation

The primary advantage of k3OS is the elimination of manual post-install configuration. The system utilizes a YAML-based configuration approach, allowing users to define the state of the node before the flashing process.

• YAML Configuration: The entire identity and access control of the node are declared in a YAML file.
• MAC Address Mapping: In specific image generation scripts, such as the one provided by Sjors Gielen, the script maps configuration files to specific hardware based on the MAC address of the Raspberry Pi.
• File Naming Convention: Configuration files must be placed in the config/ folder and named according to the MAC address (e.g., config/6A:4A:19:32:8A:50.yaml).

Within the config.yaml, users can define essential parameters:

• Hostname: The specific name the Pi will assume within the cluster (e.g., hostname: k3s-1).
• SSH Access: Public keys are defined in the configuration to allow secure remote access. K3OS primarily utilizes a single user named rancher. The root account is disabled by design, lacks a password, and is never assigned an SSH key.
• SSH Key Integration: SSH keys can be integrated directly from GitHub user accounts using the format github:${USERNAME}.

To generate these images, developers utilize the picl-k3OS-image-generator repository. The process involves cloning the repository and executing the build script:

./build-image.sh raspberrypi

This command generates a flashable image, such as picl-k3OS-v0.10.0-raspberrypi.img, which can then be written to an SD card using tools like Balena Etcher. Upon the first boot, the image performs several automated initialization tasks, meaning the device may take several minutes before it responds to network requests.

Comparative Analysis of k3OS versus General Purpose OS

The transition from Raspberry Pi OS or Ubuntu to k3OS is often driven by specific technical failures encountered in general-purpose environments.

A common failure point in Raspberry Pi OS is the resolution of DNS inside pods. Users have reported that while the first node may function, subsequent nodes often fail to resolve DNS, necessitating a complete system wipe and a transition to k3OS. Similarly, Ubuntu updates on Raspberry Pi 3 or 4 have been reported to make the system nearly unusable, resulting in frequent pod failures. k3OS resolves these issues by stripping away the unnecessary layers of a general-purpose OS and focusing solely on the requirements of the K3s binary.

Technical Implementation of K3s on Raspberry Pi 5 and 4

For those opting for a manual installation on Raspberry Pi OS (specifically on the Raspberry Pi 5), several critical system-level modifications are required before K3s can be deployed.

System Preparation and Cgroup Configuration

Kubernetes requires cgroup support for memory management, which is not enabled by default on Raspberry Pi OS. Failure to enable this will prevent the K3s service from functioning correctly.

The following steps are required to enable cgroup memory support:

1. Edit the boot configuration file:
   sudo nano /boot/firmware/cmdline.txt

2. Append the following parameters to the end of the existing line, ensuring no new lines are created:
   cgroup_memory=1 cgroup_enable=memory

3. Save the file and restart the system:
   sudo reboot

Additionally, the system must be updated and essential networking tools installed:

sudo apt update -y
sudo apt install -y curl wget

Manual K3s Installation Process

Once the kernel is prepared, the installation is performed via the official K3s installation script.

For the primary server node (Control Plane):
curl -sfL https://get.k3s.io | sh -s - server --cluster-init

To add subsequent nodes to create a High Availability (HA) cluster where master nodes are schedulable:
curl -sfL https://get.k3s.io | sh -s - server --server https://<ip or hostname of server1>:6443

For adding standard worker nodes, the server token must first be retrieved from the primary node:
sudo cat /var/lib/rancher/k3s/server/node-token

Then, the worker node is joined using the following command:
curl -sfL https://get.k3s.io | K3S_URL=https://<SERVER_IP>:6443 K3S_TOKEN=<TOKEN> sh -

Verification and Service Management

After installation, the health of the cluster is verified using the integrated kubectl tool.

To check the status of the K3s service:
sudo systemctl status k3s

To verify that the nodes are in a Ready state:
sudo kubectl get nodes

Advanced k3OS Management and Upgrades

k3OS provides specific mechanisms for updating the system root filesystem and the kernel. While the distribution is designed to be immutable in many respects, upgrade scripts are available in the system directory.

Upgrade Procedures

As of version v0.9.0, certain upgrade scripts have been deprecated but remain available at /usr/share/rancher/k3os/scripts. To perform an upgrade, the user must first specify the desired version:

export K3OS_VERSION=v0.10.0

Then, the root filesystem is upgraded:
/usr/share/rancher/k3os/scripts/k3os-upgrade-rootfs

Followed by the kernel upgrade attempt:
/usr/share/rancher/k3os/scripts/k3os-upgrade-kernel

A critical nuance of the k3OS architecture is the "Overlay Installation." When k3OS is installed as an overlay on an existing distribution like Raspbian, the original distro kernel is retained. In these instances, the k3os-upgrade-kernel script will issue a warning and perform no action, as it cannot override the host kernel.

Building k3OS from Source

For developers needing to customize the OS, k3OS can be built from source using Docker.

1. Clone the k3OS repository.
2. Run the make command:
   make

The resulting artifacts are stored in the ./dist/artifacts directory. For those testing on Linux, a VM instance of k3OS can be launched in the terminal via:
./scripts/run

To exit the QEMU console, the user must press CTRL+a c and then q.

The kernel itself is maintained in a separate repository (https://github.com/rancher/k3os-kernel), where the same Docker-based make process is used for compilation.

Control Plane Database Externalization

One of the most significant bottlenecks in Raspberry Pi clusters is the reliance on SD cards for the etcd database. The high frequency of writes required by etcd can lead to severe performance degradation and premature SD card failure.

External Database Options

K3s supports several external database engines to offload the control plane state:

• PostgreSQL
• MySQL
• MariaDB
• External etcd

By running one of these databases on a more powerful machine—such as a Mac running Docker—the Raspberry Pi nodes are relieved of the intensive database management overhead. This is particularly crucial for low-power devices like the Raspberry Pi Zero 2, where the k3s daemon may fail to start entirely due to memory limitations. By externalizing the DB, the Pi nodes function primarily as execution environments for pods rather than state managers for the cluster.

Deployment Analysis and System Evaluation

The deployment of k3OS on Raspberry Pi 4 represents a strategic move away from general-purpose operating systems toward a specialized, immutable-infrastructure approach. The transition from an imperative installation (running scripts on a live OS) to a declarative one (flashing a pre-configured image) significantly reduces the surface area for configuration errors.

The most profound impact is seen in the networking layer. By using k3OS, the complexities of DNS resolution within pods—which often plague Raspberry Pi OS deployments—are handled at the image level. Furthermore, the ability to map configurations to MAC addresses allows for the rapid scaling of clusters without the need for manual intervention on each node.

However, k3OS is not without its limitations. As a beta operating system, the documentation is acknowledged as being imperfect. Users may find that certain utilities, such as rsyslog, are not present by default and must be added by altering the base image. Additionally, the system lacks a native, simple method for setting a static IP address, necessitating reliance on DHCP or external network management.

Ultimately, the efficacy of a k3os-powered Raspberry Pi cluster depends on the balance between hardware capabilities and software configuration. While the Raspberry Pi 4 provides sufficient RAM (4GB/8GB) to handle the workload, the strategic decision to externalize the control plane database remains the single most effective way to ensure long-term stability and performance, especially when utilizing microSD storage.

Sources

1. Chris Woolum
2. PicoCluster
3. Andre Veiga
4. Rancher k3os GitHub
5. Keloran