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
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 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-standardThe 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-ssdUsing 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: 30GiThis 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
StorageClassobject as default; - Making sure that the
DefaultStorageClassadmission 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.