Every container gets a writable layer on top of its image. Writes go there. When the container is removed, the layer is removed.

Pod db-0
  container postgres
    image layers   (read-only)
    writable layer (deleted with the container)

A Deployment replacing a Pod, a node reboot, a crash loop: each one discards the layer. A database in a bare container loses its data on the first restart.

Volumes

A volume is a directory mounted into a container from outside the writable layer. The source decides how long the data lives:

emptyDir   lives as long as the Pod. Deleted with the Pod.
hostPath   a path on the node. Survives the Pod, tied to that node.
PVC        a claim on cluster storage. Survives the Pod and the node.

The claim model

Three objects separate "I need 1 GB" from "here is a disk":

Pod ──mounts──▶ PersistentVolumeClaim ──bound to──▶ PersistentVolume ──backed by──▶ disk
                (namespaced, 1Gi RWO)               (cluster-scoped, 2Gi RWO)

• PersistentVolume (PV): a piece of storage in the cluster. Cluster-scoped. Has a capacity, access modes, a reclaim policy and a backing (hostPath, local disk, cloud volume).
• PersistentVolumeClaim (PVC): a request for storage in a namespace. Has a size and access modes. Binds to one PV.
• StorageClass: a recipe for creating PVs on demand. Names a provisioner, a reclaim policy and a binding mode.

The Pod names the claim. The claim is bound to a volume. The Pod never names the volume.

Access modes

ReadWriteOnce  (RWO)  one node can mount it read-write
ReadOnlyMany   (ROX)  many nodes can mount it read-only
ReadWriteMany  (RWX)  many nodes can mount it read-write

A claim binds only to a volume that offers every mode the claim asks for.

Reclaim policies

What happens to a PV when its claim is deleted:

Delete   the PV and the data are removed
Retain   the PV stays, phase Released, data intact, claimRef still set

Static vs dynamic

Static: an administrator creates PVs by hand. A PVC with storageClassName: "" binds to the first Available PV that fits.

Dynamic: a PVC names a StorageClass (or gets the default). The class's provisioner creates a PV sized to the claim.

This cluster has one class:

kubectl get storageclass

local-path is the default. Its binding mode is WaitForFirstConsumer: a claim on it stays Pending until a Pod mounts it, then a PV is created and bound.