What are Kubernetes volumes, their types and concepts behind them? We’ll cover all of that below. In addition, we’ll see practical demo of using Kubernetes volumes.
Kubernetes volumes answer below questions along with many others.
- what happens to the data written by containers in a pod after the pod stops?
- how do containers running in a pod share data?
You may already know that
Kubernetes uses volumes to store data of apps. You may also be familiar with
docker volumes and how they work. Are there more storage options in
Kubernetes world? What kinds of volumes exist? Keep reading to find out. As always, we’ll see a practical demo as well.
Kubernetes volumes explained
So we know that
Kubernetes volumes store data of apps running in container pods. Examples of data could be app logs or files the app shares with other apps. App example could be containerized database. But what
Kubernetes volume actually is? On the most basic level it’s a directory with data to which container running in a pod has access to.
What happens to data if a container stops or crashes? Usually, the data is lost unless it’s mapped using volumes to the host.
Kubernetes solves this problem using volumes as well. There a lot volume types. And of course, volumes specific behavior may vary. Though, if we select the most common type for sharing data between containers in a pod –
emptyDir, the data will survive. It’s true even if there’s only one container in a pod and it crashes. Of course, the data is safe only as long as the pod is alive.
What if we want data persistence beyond pod life-cycle? For that
Persistent volume. What is it? It’s
Kubernetes cluster storage unit. Its data is safe even if the pod using it ceases to exist.
There are a lot of
Kubernetes volume types. The most common ones are:
Kubernetesnode path to containers in a pod
emptyDir– shares data between containers in a pod.
nfs– shares data with a pod using network storage.
persistentVolumeClaim– mounts a
PersistentVolumeinto a pod.
I would cover below important volume types in future posts.
It’s time to see
Kubernetes volumes demo.
Kubernetes volumes demo
Install on your machine:
Mount host data to containers in a pod
Imagine a situation when a pod or more precisely its containers need to access host data. For example, we might need to run docker client inside docker container which runs in a pod. To that end we need to mount docker socket from the host to the container. We’ll use
hostPath volume type for that.
Let’s see a real-world example when Jenkins needs to run docker client commands inside docker agents. You wonder why it might be needed? For instance, for pushing an image to a registry in CI/CD pipeline. We have seen such example when we explored CI/CD using Jenkins and Docker.
Mount Persistent Volume to a pod
We’ll see also how
Jenkins data is persisted across
Jenkins restarts thanks to
PersistentVolumeClaim. It mounts
Kubernetes volumes in action step by step
docker_in_jenkinsrepository from my GitHub and
minikubecluster and initialize
minikube start --profile custom skaffold config set --global local-cluster true # run below in any new terminal to cause kubectl commands be configured to run against minikube cluster eval $(minikube -p custom docker-env)
scaffold runto build and apply the manifests to
- it will take some time to build the image in
minikubevirtual machine and deploy the manifests to
- make sure
jenkinsis running using
kubectl get all | grep jenkins. You can see that
Jenkinsis deployed. Its
ReplicaSetto manage the pods. And it’s exposed as
Service. You may find Kubernetes Services, Kubernetes Deployments, ReplicaSets and Kubernetes Pods demos useful.
- let’s now access
Jenkins. To that end, we’ll use port forwarding:
kubectl port-forward svc/jenkins 8080:8080
- navigate to https://localhost:8080 and you should see
Jenkinsis running and configured for us. Next, login as
- now, create new Jenkins job which runs CI/CD flow that uses docker commands. I’ll follow Running CI/CD pipeline in Jenkins to create
todobackendjob. Now run the job. After a while the full CI/CD flow finishes.
Kubernetes hostPath volume usage
- so we have Jenkins running in docker container. And docker commands run inside Jenkins container when the job runs. In order to succeed, docker client in Jenkins container needed access to docker socket from the docker host. How did it work? Right. Thanks to
Kubernetesvolume. More specifically,
docker-socketvolume of type
hostPath. Note that in our case, docker host is
- name: docker-socket hostPath: path: /var/run/docker.sock type: Socket
- the volume is mounted at
/var/run/docker.sockinside Jenkins container.
- mountPath: /var/run/docker.sock name: docker-socket
- do you still have doubts? Enter
Jenkinscontainer and run sample docker command inside. Run below commands in a new terminal in order to keep port forwarding running in the first terminal.
$ kubectl get pod | grep jenkins jenkins-55b4bddcc8-kk7th 1/1 Running 0 24m $ kubectl exec -it jenkins-55b4bddcc8-kk7th bash # inside jenkins container root@jenkins-55b4bddcc8-kk7th:/# docker ps | grep jenkins 677ccc4d1f0d 6460b0620ef3 "/usr/local/bin/entr…" 25 minutes ago Up 25 minutes k8s_jenkins_jenkins-55b4bddcc8-kk7th_default_4de858bf-5eb8-4f5c-b6ae-50ee98270de0_0
- what about
Jenkinsdata like job we created, installed plugins, etc.. Will we lose it when we restart
Jenkins? We’d rather not 🙂 Let’s see what happens after we restart
$ kubectl get deployments.apps | grep jenkins jenkins 1/1 1 1 17h $ kubectl rollout restart deployment jenkins deployment.apps/jenkins restarted
- login to
JenkinsUI and magically
todojob we created is still there.
- of course, data is not lost even after restart. How did we achieve that? We used
Kubernetesvolume of type
volumes: - name: jenkins-data persistentVolumeClaim: claimName: jenkins-data
In addition, we mounted to the pod using:
volumeMounts: - mountPath: /var/jenkins_home name: jenkins-data
As you can see, in order to use volumes we added
Jenkins deployment in pod template spec. In addition, we specified which volumes to mount to pod containers using
To make sure you always get it right refer to
Kubernetes API reference.
It also provides the most succinct line about volumes I ever saw:
Volumes provide filesystem external to the Container. Maybe shared across Containers within the same Pod and have a lifetime persisting beyond a Container or Pod.
- Finally, stop port forwarding and our cluster.
$ minikube stop --profile custom
That’s it about Kubernetes volumes. Please, note that as professional Kubernetes admin, you’d probably not create Kubernetes volumes manually. You’d rather use Kubernetes storage dynamic provisioning.
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