How to reference a local volume in Kind (kubernetes in docker)

Question

I'd like to set a Persistent Volume (pv and pvc), shared by pods in Kind cluster. But I need to keep the data persisted in my laptop (host server) as well, so the volume's path should be something in my laptop, which I can directly access it.

If I delete the kind cluster, the volume should be persisted, not be destroyed.

I hope to easily add or update in that volume or copy files out of it from my host laptop.

How can I let the pods know it in KIND cluster?

Paste my kind.yaml for your refernce

$ kind
kind: Cluster
apiVersion: kind.x-k8s.io/v1alpha4
nodes:
- role: control-plane
- role: worker
- role: worker
- role: worker

Answer 1

When you create your kind cluster you can specify host directories to be mounted on a virtual node. If you do that, then you can configure volumes with hostPath storage, and they will refer to the mount paths on the node.

So you would create a kind config file:

apiVersion: kind.x-k8s.io/v1alpha4
kind: Cluster
nodes:
  - role: control-plane
    extraMounts:
      - hostPath: /home/bill/work/foo
        containerPath: /foo

and then run

kind create cluster --config kind-config.yaml

to create the cluster.

In your Kubernetes YAML file, you need to mount that containerPath as a "host path" on the node. A pod spec might contain in part:

volumes:
  - name: foo
    hostPath:
      path: /foo  # matches kind containerPath:
containers:
  - name: foo
    volumeMounts:
      - name: foo
        mountPath: /data  # in the container filesystem

Note that this setup is extremely specific to kind. Host paths aren't reliable storage in general: you can't control which node a pod gets scheduled on, and both pods and nodes can get deleted in real-world clusters. In some hosted setups (AWS EKS, Google GKE) you may not be able to control the host content at all.

You might revisit your application design to minimize the need for "files" as first-class objects. Rather than "update the volume" consider deploying a new Docker image with updated content; rather than "copy files out" consider an HTTP service you can expose through an ingress controller.

Answer 2

I would like to add that to minimise the specific configuration to Kind you should use pv / pvc this way the configuration on a real cluster will only differ in the definition of pv.

So if you configure extraMounts on your Kind cluster:

apiVersion: kind.x-k8s.io/v1alpha4
kind: Cluster
nodes:
- role: control-plane
  extraMounts:
    - hostPath: /home/bill/work/www
      containerPath: /www

Then on that cluster create PV and PVC:

---
apiVersion: v1
kind: PersistentVolume
metadata:
  name: pv-www
spec:
  storageClassName: standard
  accessModes:
    - ReadWriteOnce
  capacity:
    storage: 2Gi
  hostPath:
    path: /www/
---
apiVersion: v1
kind: PersistentVolumeClaim
metadata:
  name: pvc-www
spec:
  volumeName: pv-www
  accessModes:
    - ReadWriteOnce
  resources:
    requests:
      storage: 1Gi

After that you can use it in deployment like this:

---
apiVersion: apps/v1
kind: Deployment
metadata:
  name: nginx-deployment
  labels:
    app: nginx
spec:
  replicas: 3
  selector:
    matchLabels:
      app: nginx
  template:
    metadata:
      labels:
        app: nginx
    spec:
      volumes:
      - name: www
        persistentVolumeClaim:
          claimName: pvc-www
      containers:
      - name: nginx
        image: nginx:1.14.2
        volumeMounts:
        - name: www
          mountPath: /var/www

As a result your local /home/bill/work/www will be mounted to /var/www inside containers.