Docker Image Management in Kubernetes with k3s

Managing container images effectively is crucial for Kubernetes deployments to ensure smooth operations, resource efficiency, and security. In this blog post, we'll explore image management in Kubernetes with k3s, a lightweight Kubernetes distribution, covering image creation, storage, and best practices.

Understanding Image Management in Kubernetes

In Kubernetes, container images are the building blocks of applications. Image management involves creating, storing, and distributing these images to Kubernetes clusters.

Image Creation

Container images are typically created using Dockerfiles or other container build tools. These images contain the application code, dependencies, and runtime environment required for execution. With k3s, you can use familiar Docker commands to build images:

docker build -t image_name:tag_name .

save the image into the container

docker save image_name:tag_name | sudo k3s ctr images import -

containers:
- name: my-container
  image: my-image:v1
  imagePullPolicy: IfNotPresent

Image Pull Policy

The imagePullPolicy specification lets you specify how you want the Kubelet to pull an image if there’s any change (restart, update, etc.) to a Pod. When using the imagePullPolicy specification, you have three options:

1. IfNotPresent: If you set the imagePullPolicy to IfNotPresent, Kubernetes will only pull the image when it doesn’t already exist on the node.
2. Always: With your imagePullPolicy set to Always, Kubernetes will always pull the latest version of the image from the container registry.
3. Never: If you set the imagePullPolicy to Never, there will be no attempts to pull the image.

Default image pull policy

You may ask what happens when the imagePullPolicy specification isn’t defined in a manifest file — just like in the above manifest file. Well, in that case:

• If the image tag is :latest, the imagePullPolicy will be automatically set to Always.
• If the image tag isn’t :latest, the imagePullPolicy will be automatically set to IfNotPresent.
• And if you don’t set any image tag, the imagePullPolicy will be automatically set to latest image and Always value.

ImagePullBackOff

When a kubelet starts creating containers for a Pod, it might be possible a container gets stuck in its Waiting state because of the ImagePullBackOff error status.

The ImagePullBackOff error means that a container couldn’t start because Kubernetes could not pull the image for reasons such as:

• An invalid image name,
• Pulling from a private registry without an imagePullSecret or using an incorrect imagePullSecret, etc. The BackOff part indicates that Kubernetes will keep attempting to pull the image, with an increasing “back-off“ delay.

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some Important commands for k3s docker

1. Show all images on the k3s

sudo k3s crictl images

2. Delete some image/s on the k3s

k3s crictl rmi NAME:TAG

3. Show process

k3s crictl ps -a

k3s crictl ps

4. See all pods

k3s crictl pods

5. See stats

k3s crictl stats

6. Import data

podman/docker save NAME:TAG | k3s ctr images import -