Dynamic volume provisioning

Dynamic volume provisioning allows storage volumes to be created on-demand. Without dynamic provisioning, cluster administrators have to create new storage volumes manually (using cloud or storage providers) and the corresponding PersistentVolume objects for the storage to be available in Kubernetes.

Dynamic volume provisioning is enabled by default on the CAST AI cluster.

Overview

Each CAST AI cluster is pre-configured with the default StorageClass that handles volume requests.

» kubectl get sc
NAME                           PROVISIONER           RECLAIMPOLICY   VOLUMEBINDINGMODE      ALLOWVOLUMEEXPANSION   AGE
cast-block-storage (default)   storage.csi.cast.ai   Delete          WaitForFirstConsumer   true                   2m18s

The binding mode WaitForFirstConsumer will delay the binding and provisioning of a PersistentVolume until a Pod using the PVC is created. Meaning, the volume will be created and attached to the Node on which a Pod using the PVC will be run.

In the case of a Pod replicated across multiple clouds, volumes will be distributed across clouds as well. This will limit Pod scheduling only to the nodes of the same cloud since to reschedule a Pod to a different cloud service, the volume must be replicated to that cloud.

This limitation will be removed by the cross-cloud volume replication feature which is not available at the moment.

Deleting a cluster will delete all the volumes that were provisioned dynamically.

Using dynamic volumes

Creating persitent volume claim (PVC)

Users can request dynamically provisioned storage by simply creating PersistentVolumeClaim and a Pod that will use it.

apiVersion: v1
kind: PersistentVolumeClaim
metadata:
  name: example-claim
spec:
  accessModes:
    - ReadWriteOnce
  resources:
    requests:
      storage: 50Gi

Pod example:

apiVersion: v1
kind: Pod
metadata:
  name: app
spec:
  containers:
    - name: app
      image: centos
      command: ["/bin/sh"]
      args: ["-c", "while true; do echo $(date -u) >> /data/out.txt; sleep 5; done"]
      volumeMounts:
        - name: persistent-storage
          mountPath: /data
  volumes:
    - name: persistent-storage
      persistentVolumeClaim:
        claimName: example-claim

This claim results in a Persistent Disk being automatically provisioned. When the claim is deleted, the volume is deleted as well.

Volume claim templates

Additionally, having StatefulSet user can define volumeClaimTemplates to provision volumes without creating PVC beforehand.

apiVersion: v1
kind: Service
metadata:
  name: nginx
  labels:
    app: nginx
spec:
  ports:
  - port: 80
    name: web
  clusterIP: None
  selector:
    app: nginx
---
apiVersion: apps/v1
kind: StatefulSet
metadata:
  name: web
spec:
  selector:
    matchLabels:
      app: nginx
  serviceName: "nginx"
  replicas: 3
  template:
    metadata:
      labels:
        app: nginx
    spec:
      containers:
      - name: nginx
        image: k8s.gcr.io/nginx-slim:0.8
        ports:
        - containerPort: 80
          name: web
        volumeMounts:
        - name: www
          mountPath: /usr/share/nginx/html
  volumeClaimTemplates:
  - metadata:
      name: www
    spec:
      accessModes: [ "ReadWriteOnce" ]
      resources:
        requests:
          storage: 1Gi

This will result in dynamic PVC for each StatefulSet pod.

» kubectl get pvc
NAME            STATUS   VOLUME                                     CAPACITY   ACCESS MODES   STORAGECLASS         AGE
www-web-0       Bound    pvc-3b550fd0-4b79-449d-b1cf-f51264f975fc   1Gi        RWO            cast-block-storage   3m11s
www-web-1       Bound    pvc-0c7470a8-cc59-49d4-b2ca-5d3db45c1b60   1Gi        RWO            cast-block-storage   2m41s
www-web-2       Bound    pvc-70c341cc-fa1c-471a-882a-e46225e1824f   1Gi        RWO            cast-block-storage   2m18s

Deleting a StatefulSet will delete all provisioned volumes.

Resizing PVC

Any PVC created using cast-block-storage StorageClass can be edited to request more space. Kubernetes will interpret a change to the storage field as a request for more space. This will trigger automatic volume resizing.

» kubectl edit pvc www-web-0

Change storage field as shown below:

# www-web-0...
spec:
  accessModes:
  - ReadWriteOnce
  resources:
    requests:
      storage: 10Gi # new storage size
  storageClassName: cast-block-storage
# www-web-0...

After storage is resized successfully, we can observe new PVC capacity:

k get pvc www-web-0
NAME        STATUS   VOLUME                                     CAPACITY   ACCESS MODES   STORAGECLASS         AGE
www-web-0   Bound    pvc-edd59e56-cb22-41b6-a075-ab8820f222b8   10Gi       RWO            cast-block-storage   4m57s