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 we will cover elsewhere.

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

$ kubectl create -f https://raw.githubusercontent.com/mhausenblas/kbe/master/specs/volumes/pod.yaml

$ kubectl describe pod sharevol
Name:                   sharevol
Namespace:              default
...
Volumes:
  xchange:
    Type:       EmptyDir (a temporary directory that shares a pod's lifetime)
    Medium:

We first exec into one of the containers in the pod, c1, check the volume mount and generate some data:

$ kubectl exec sharevol -c c1 -i -t -- bash
[root@sharevol /]# mount | grep xchange
/dev/sda1 on /tmp/xchange type ext4 (rw,relatime,data=ordered)
[root@sharevol /]# echo 'some data' > /tmp/xchange/data

When we now exec into c2, the second container running in the pod, we can see the volume mounted at /tmp/data and are able to read the data created in the previous step:

$ kubectl exec sharevol -c c2 -i -t -- bash
[root@sharevol /]# mount | grep /tmp/data
/dev/sda1 on /tmp/data type ext4 (rw,relatime,data=ordered)
[root@sharevol /]# cat /tmp/data/data
some data

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.

Previous | Next