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Objective 4: Storage

4.1 Understand Storage Classes, Persistent Volumes

Storage Classes

  • Reclaim Policy: PersistentVolumes that are dynamically created by a StorageClass will have the reclaim policy specified in the reclaimPolicy field of the class
    • Delete: When PersistentVolumeClaim is deleted, also deletes PersistentVolume and underlying storage object
    • Retain: When PersistentVolumeClaim is deleted, PersistentVolume remains and volume is "released"
  • Volume Binding Mode:
    • Immediate: By default, the Immediate mode indicates that volume binding and dynamic provisioning occurs once the PersistentVolumeClaim is created
    • WaitForFirstConsumer: Delay the binding and provisioning of a PersistentVolume until a Pod using the PersistentVolumeClaim is created
      • Supported by AWSElasticBlockStore, GCEPersistentDisk, and AzureDisk
  • Allow Volume Expansion: Allow volumes to be expanded
    • Note: It is not possible to reduce the size of a PersistentVolume
  • Default Storage Class: A default storage class is used when a PersistentVolumeClaim does not specify the storage class
    • Can be handy when a single default services all pod volumes
  • Provisioner
    • Determines the volume plugin to use for provisioning PVs.
    • Example: gke-pd, azure-disk

View all storage classes

kubectl get storageclass or kubectl get sc

NAME                 PROVISIONER            RECLAIMPOLICY   VOLUMEBINDINGMODE   ALLOWVOLUMEEXPANSION   AGE
standard (default)   kubernetes.io/gce-pd   Delete          Immediate           true                   25h

View the storage class in yaml format

kubectl get sc standard -o yaml

allowVolumeExpansion: true
apiVersion: storage.k8s.io/v1
kind: StorageClass
metadata:
  name: standard
parameters:
  type: pd-standard
provisioner: kubernetes.io/gce-pd
reclaimPolicy: Delete
volumeBindingMode: Immediate

Make a custom storage class using the yaml configuration below and save it as speedyssdclass.yaml

allowVolumeExpansion: true
apiVersion: storage.k8s.io/v1
kind: StorageClass
metadata:
  name: speedyssdclass
parameters:
  type: pd-ssd # Note: This will use SSD backed disks
  fstype: ext4
  replication-type: none
provisioner: kubernetes.io/gce-pd
reclaimPolicy: Delete
volumeBindingMode: Immediate

Apply the storage class configuration to the cluster

kubectl apply -f speedyssdclass.yaml

storageclass.storage.k8s.io/speedyssdclass created

Get the storage classes

kubectl get sc

NAME                 PROVISIONER            RECLAIMPOLICY   VOLUMEBINDINGMODE      ALLOWVOLUMEEXPANSION   AGE
speedyssdclass       kubernetes.io/gce-pd   Retain          WaitForFirstConsumer   true                   5m19s
standard (default)   kubernetes.io/gce-pd   Delete          Immediate              true                   8d

Persistent Volumes

View a persistent volume in yaml format

kubectl get pv pvc-d2f6e37e-277f-4b7b-8725-542609f1dea4 -o yaml

apiVersion: v1
kind: PersistentVolume
metadata:
  name: pvc-d2f6e37e-277f-4b7b-8725-542609f1dea4
spec:
  accessModes:
    - ReadWriteOnce
  capacity:
    storage: 1Gi
  persistentVolumeReclaimPolicy: Delete
  storageClassName: standard
  volumeMode: Filesystem

Create a new disk named pv100 in Google Cloud to be used as a persistent volume

Note: Use the zone of your GKE cluster

gcloud compute disks create pv100 --size 10GiB --zone=us-central1-c

Make a custom persistent volume using the yaml configuration below and save it as pv100.yaml

apiVersion: v1
kind: PersistentVolume
metadata:
  name: pv100
spec:
  accessModes:
    - ReadWriteOnce
  capacity:
    storage: 1Gi
  persistentVolumeReclaimPolicy: Delete
  storageClassName: standard
  volumeMode: Filesystem
  gcePersistentDisk: # This section is required since we are not using a Storage Class
    fsType: ext4
    pdName: pv100

Apply the persistent volume to the cluster

kubectl apply -f pv100.yaml

persistentvolume/pv100 created

Get the persistent volume and notice that it has a status of Available since there is no PersistentVolumeClaim to bind against

kubectl get pv pv100

NAME    CAPACITY   ACCESS MODES   RECLAIM POLICY   STATUS      CLAIM   STORAGECLASS   REASON   AGE
pv100   1Gi        RWO            Delete           Available           standard                2m51s

4.2 Understand Volume Mode, Access Modes And Reclaim Policies For Volumes

Volume Mode

  • Filesystem: Kubernetes formats the volume and presents it to a specified mount point.
    • If the volume is backed by a block device and the device is empty, Kuberneretes creates a filesystem on the device before mounting it for the first time.
  • Block: Kubernetes exposes a raw block device to the container.
    • Improved time to usage and perhaps performance.
    • The container must know what to do with the device; there is no filesystem.
  • Defined in .spec.volumeMode for a PersistentVolumeClaim.

View the volume mode for persistent volume claims using the -o wide to see the VOLUMEMODE column

kubectl get pvc -o wide

NAME        STATUS   VOLUME                                     CAPACITY   ACCESS MODES   STORAGECLASS   AGE   VOLUMEMODE
www-web-0   Bound    pvc-f3e92637-7e0d-46a3-ad87-ef1275bb5a72   1Gi        RWO            standard       19m   Filesystem
www-web-1   Bound    pvc-d2f6e37e-277f-4b7b-8725-542609f1dea4   1Gi        RWO            standard       19m   Filesystem

Access Modes

  • ReadWriteOnce (RWO): can be mounted as read-write by a single node
  • ReadOnlyMany (ROX): can be mounted as read-only by many nodes
  • ReadWriteMany (RWX): can be mounted as read-write by many nodes
  • Defined in .spec.accessModes for a PersistentVolumeClaim and PersistentVolume

View the access mode for persistent volume claims

kubectl get pvc

NAME        STATUS   VOLUME                                     CAPACITY   ACCESS MODES   STORAGECLASS   AGE
www-web-0   Bound    pvc-f3e92637-7e0d-46a3-ad87-ef1275bb5a72   1Gi        RWO            standard       28m
www-web-1   Bound    pvc-d2f6e37e-277f-4b7b-8725-542609f1dea4   1Gi        RWO            standard       27m

Reclaim Policies

  • Reclaim Policy: PersistentVolumes that are dynamically created by a StorageClass will have the reclaim policy specified in the reclaimPolicy field of the class
    • Delete: When PersistentVolumeClaim is deleted, also deletes PersistentVolume and underlying storage object
    • Retain: When PersistentVolumeClaim is deleted, PersistentVolume remains and volume is "released"
  • Change the Reclaim Policy of a PersistentVolume
  • Defined in .spec.persistentVolumeReclaimPolicy for PersistentVolume.

View the reclaim policy set on persistent volumes

kubectl get pv

NAME                                       CAPACITY   ACCESS MODES   RECLAIM POLICY   STATUS   CLAIM               STORAGECLASS   REASON   AGE
pvc-d2f6e37e-277f-4b7b-8725-542609f1dea4   1Gi        RWO            Delete           Bound    default/www-web-1   standard                45m
pvc-f3e92637-7e0d-46a3-ad87-ef1275bb5a72   1Gi        RWO            Delete           Bound    default/www-web-0   standard                45m

4.3 Understand Persistent Volume Claims Primitive

Make a custom persistent volume claim using the yaml configuration below and save it as pvc01.yaml

apiVersion: v1
kind: PersistentVolumeClaim
metadata:
  name: pvc01
spec:
  storageClassName: standard
  accessModes:
    - ReadWriteOnce
  resources:
    requests:
      storage: 3Gi

Apply the persistent volume claim

kubectl apply -f pvc01.yaml

persistentvolumeclaim/pvc01 created

Get the persistent volume claim

kubectl get pvc pvc01

NAME    STATUS   VOLUME                                     CAPACITY   ACCESS MODES   STORAGECLASS   AGE
pvc01   Bound    pvc-9f2e7c5d-b64c-467e-bba6-86ccb333d981   3Gi        RWO            standard       5m19s

4.4 Know How To Configure Applications With Persistent Storage

Create a new yaml file using the configuration below and save it as pv-pod.yaml

Note: Make sure to create pvc01 in this earlier step

apiVersion: v1
kind: Pod
metadata:
  name: pv-pod
spec:
  volumes:
    - name: pv-pod-storage # The name of the volume, used by .spec.containers.volumeMounts.name
      persistentVolumeClaim:
        claimName: pvc01 # This pvc was created in an earlier step
  containers:
    - name: pv-pod-container
      image: nginx
      ports:
        - containerPort: 80
          name: "http-server"
      volumeMounts:
        - mountPath: "/usr/share/nginx/html"
          name: pv-pod-storage # This refers back to .spec.volumes.name

Apply the pod

kubectl apply -f pv-pod.yaml

pod/pv-pod created

Watch the pod provisioning process

kubectl get pod -w pv-pod

NAME     READY   STATUS    RESTARTS   AGE
pv-pod   1/1     Running   0          30s

View the binding on pvc01

kubectl describe pvc pvc01

Name:          pvc01
Namespace:     default
StorageClass:  standard
Status:        Bound
Volume:        pvc-9f2e7c5d-b64c-467e-bba6-86ccb333d981
Labels:        <none>
Annotations:   pv.kubernetes.io/bind-completed: yes
               pv.kubernetes.io/bound-by-controller: yes
               volume.beta.kubernetes.io/storage-provisioner: kubernetes.io/gce-pd
Finalizers:    [kubernetes.io/pvc-protection]
Capacity:      3Gi
Access Modes:  RWO
VolumeMode:    Filesystem
Mounted By:    pv-pod # Here it is!
Events:
  Type    Reason                 Age   From                         Message
  ----    ------                 ----  ----                         -------
  Normal  ProvisioningSucceeded  36m   persistentvolume-controller  Successfully provisioned volume pvc-9f2e7c5d-b64c-467e-bba6-86ccb333d981 using kubernetes.io/gce-pd