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Dynamic Volume Provisioning

Dynamic volume provisioning allows storage volumes to be created on-demand. Without dynamic provisioning, cluster administrators have to manually make calls to their cloud or storage provider to create new storage volumes, and then create PersistentVolume objects to represent them in Kubernetes. The dynamic provisioning feature eliminates the need for cluster administrators to pre-provision storage. Instead, it automatically provisions storage when it is requested by users.

• Background
• Enabling Dynamic Provisioning
• Using Dynamic Provisioning
• Defaulting Behavior

Background

The implementation of dynamic volume provisioning is based on the API object StorageClass from the API group storage.k8s.io. A cluster administrator can define as many StorageClass objects as needed, each specifying a volume plugin (aka provisioner) that provisions a volume and the set of parameters to pass to that provisioner when provisioning. A cluster administrator can define and expose multiple flavors of storage (from the same or different storage systems) within a cluster, each with a custom set of parameters. This design also ensures that end users don’t have to worry about the the complexity and nuances of how storage is provisioned, but still have the ability to select from multiple storage options.

More information on storage classes can be found here.

Enabling Dynamic Provisioning

To enable dynamic provisioning, a cluster administrator needs to pre-create one or more StorageClass objects for users. StorageClass objects define which provisioner should be used and what parameters should be passed to that provisioner when dynamic provisioning is invoked. The following manifest creates a storage class “slow” which provisions standard disk-like persistent disks.

apiVersion: storage.k8s.io/v1
kind: StorageClass
metadata:
  name: slow
provisioner: kubernetes.io/gce-pd
parameters:
  type: pd-standard

The following manifest creates a storage class “fast” which provisions SSD-like persistent disks.

apiVersion: storage.k8s.io/v1
kind: StorageClass
metadata:
  name: fast
provisioner: kubernetes.io/gce-pd
parameters:
  type: pd-ssd

Using Dynamic Provisioning

Users request dynamically provisioned storage by including a storage class in their PersistentVolumeClaim. Before Kubernetes v1.6, this was done via the volume.beta.kubernetes.io/storage-class annotation. However, this annotation is deprecated since v1.6. Users now can and should instead use the storageClassName field of the PersistentVolumeClaim object. The value of this field must match the name of a StorageClass configured by the administrator (see below).

To select the “fast” storage class, for example, a user would create the following PersistentVolumeClaim:

apiVersion: v1
kind: PersistentVolumeClaim
metadata:
  name: claim1
spec:
  accessModes:
    - ReadWriteOnce
  storageClassName: fast
  resources:
    requests:
      storage: 30Gi

This claim results in an SSD-like Persistent Disk being automatically provisioned. When the claim is deleted, the volume is destroyed.

Defaulting Behavior

Dynamic provisioning can be enabled on a cluster such that all claims are dynamically provisioned if no storage class is specified. A cluster administrator can enable this behavior by:

• Marking one StorageClass object as default;
• Making sure that the DefaultStorageClass admission controller is enabled on the API server.

An administrator can mark a specific StorageClass as default by adding the storageclass.kubernetes.io/is-default-class annotation to it. When a default StorageClass exists in a cluster and a user creates a PersistentVolumeClaim with storageClassName unspecified, the DefaultStorageClass admission controller automatically adds the storageClassName field pointing to the default storage class.

Note that there can be at most one default storage class on a cluster, or a PersistentVolumeClaim without storageClassName explicitly specified cannot be created.