Volumes

A Kubernetes volume is essentially a directory accessible to all containers running in a pod. In contrast to the container-local filesystem, the data in volumes is preserved across container restarts. The medium backing a volume and its contents are determined by the volume type:

• node-local types such as emptyDir or hostPath
• file-sharing types such as nfs
• cloud provider-specific types like awsElasticBlockStore, azureDisk, or gcePersistentDisk
• distributed file system types, for example glusterfs or cephfs
• special-purpose types like secret, gitRepo

A special type of volume is PersistentVolume, which is covered in its own lesson.

Let’s create a pod with two containers that use an emptyDir volume to exchange data:

$ kubectl apply -f https://github.com/openshift-evangelists/kbe/blob/main/specs/volumes/pod.yaml

Volume information is displayed in the detailed output:

$ kubectl describe pod sharevol

The output below is truncated to show the relevant volume information:

Name:         sharevol
Namespace:    default
Priority:     0
Node:         minikube/192.168.39.51
...
Containers:
  c1:
    Container ID:  docker://0cfe351e5a3131d3e02ca92a4aad8ea196cde403dcbc4713329bb418e1cce144
    ...
    Mounts:
      /tmp/xchange from xchange (rw)
      /var/run/secrets/kubernetes.io/serviceaccount from default-token-sckql (ro)
  c2:
    Container ID:  docker://93eadd487c18f5fc77885b8c343dff6891c2fdbae9752160a7d5a08c2763ba9c
    ...
    Mounts:
      /tmp/data from xchange (rw)
      /var/run/secrets/kubernetes.io/serviceaccount from default-token-sckql (ro)
...
Volumes:
  xchange:
    Type:       EmptyDir (a temporary directory that shares a pod's lifetime)
    Medium:
    SizeLimit:  <unset>
...

We first connect to one of the containers in the pod, c1, to view the volume mount and generate some data:

$ kubectl exec -it sharevol -c c1 -- bash

The volume is mounted like any other Linux volume mount:

$ mount | grep xchange

Create a file in the mount that we’ll be able to access from the other container in the pod:

$ echo 'some data' > /tmp/xchange/data

When you’re finished, disconnect from the container:

$ exit

When we now connect to the container c2, the second container running in the pod, we can see the volume mounted at /tmp/data (as compared to c1 where it is mounted to /tmp/xchange) and are able to read the data created in the previous step:

$ kubectl exec -it sharevol -c c2 -- bash``

$ cat /tmp/data/data

Once again, exit from the connected container by running exit.

Note that in each container you need to decide where to mount the volume, and that for emptyDir you currently can not specify resource consumption limits.

You can remove the pod with:

$ kubectl delete pod/sharevol

As already described, this will destroy the shared volume and all its contents.

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