Hands-On Microservices with Kubernetes

In-tree and out-of-tree storage plugins

There are two types of storage plugins: in-tree and out-of-tree. In-tree means that these storage plugins are part of Kubernetes itself. In the volume clause, you refer to them by name. For example, here, a Google Compute Engine (GCE) persistent disk is configured by name. Kubernetes explicitly knows that such a volume has fields such as pdName and fsType:

volumes:  - name: test-volume    gcePersistentDisk:      pdName: my-data-disk      fsType: ext4

Take a look at the complete list of in-tree storage plugins at: https://kubernetes.io/docs/concepts/storage/persistent-volumes/#types-of-persistent-volumes.

There are several other specialized volume types, such as emptyDir, local, downwardAPI, and hostPath, that you ...