Volume

On-disk files in a container are ephemeral, which presents some problems for non-trivial applications when running in containers.

• One problem is the loss of files when a container crashes. The kubelet restarts the container but with a clean state.
• A second problem occurs when sharing files between containers running together in a Pod.

The Kubernetes volume abstraction solves both of these problems.

Kubernetes supports many types of volumes. A Pod can use any number of volume types simultaneously. Ephemeral volume types have a lifetime of a pod, but persistent volumes exist beyond the lifetime of a pod. When a pod ceases to exist, Kubernetes destroys ephemeral volumes; however, Kubernetes does not destroy persistent volumes. For any kind of volume in a given pod, data is preserved across container restarts.

At its core, a volume is a directory, possibly with some data in it, which is accessible to the containers in a pod. How that directory comes to be, the medium that backs it, and the contents of it are determined by the particular volume type used.

To use a volume, specify the volumes to provide for the Pod in .spec.volumes and declare where to mount those volumes into containers in .spec.containers[*].volumeMounts.

See also the official documentation provided by Kubernetes:

• Volume
• Persistent Volume
• Ephemeral Volume