Persistent and Ephemeral Volumes

Containers in a Pod have separate filesystems by default; a volume is a directory made available to some or all of a Pod's containers, decoupling storage from any single container's lifetime. Volumes split into two lifetimes. Ephemeral volumes, such as emptyDir, are created when the Pod is assigned to a node and exist only as long as that Pod does. Persistent volumes exist independently of any one Pod: a PersistentVolume (PV) represents an actual piece of storage, and a PersistentVolumeClaim (PVC) is a namespaced request for storage that a Pod mounts by name, without needing to know how or where that storage is actually provisioned.

For any volume type, data survives a container crash and restart within the same Pod — what changes its fate is whether the Pod itself goes away.

apiVersion: v1
kind: Pod
metadata:
  name: fetch-and-serve
spec:
  containers:
    - name: fetcher
      image: fetcher:1.0
      volumeMounts:
        - name: shared
          mountPath: /data
    - name: server
      image: nginx:1.27
      volumeMounts:
        - name: shared
          mountPath: /usr/share/nginx/html
          readOnly: true
  volumes:
    - name: shared
      emptyDir: {}

A PVC's accessModes field states how many nodes, and in what mode, the underlying storage can be mounted at once: ReadWriteOnce (RWO) allows read-write mounting by a single node; ReadOnlyMany (ROX) allows many nodes to mount the same volume read-only at the same time; ReadWriteMany (RWX) allows many nodes to mount it read-write concurrently; and ReadWriteOncePod (RWOP) is the strictest of the four, restricting the volume to exactly one Pod cluster-wide rather than one node. Not every storage backend supports every mode — a cloud block-storage volume, for instance, typically only supports RWO.

When a PVC names no storageClassName and the cluster has a StorageClass marked default, Kubernetes provisions a new PersistentVolume dynamically to satisfy the claim using that default class, rather than requiring an administrator to have pre-created one. Because a PersistentVolume is a cluster-scoped object but a PersistentVolumeClaim is namespaced, a Pod can only mount a PVC that lives in its own namespace.