Help Center/ Cloud Container Engine/ Kubernetes Basics/ Persistent Storage/ PersistentVolumes, PersistentVolumeClaims, and StorageClasses
Updated on 2024-09-24 GMT+08:00

PersistentVolumes, PersistentVolumeClaims, and StorageClasses

hostPath volumes are used for persistent storage. However, such volumes are node-specific. Data written into hostPath volumes may be different after a node restart.

If you want to read the previously written data after a pod is rebuilt and scheduled again, you can count on network storage. Typically, a cloud vendor provides at least three classes of network storage: block, file, and object storage. Kubernetes decouples how storage is provided from how it is consumed by introducing two API objects: PersistentVolume (PV) and PersistentVolumeClaim (PVC). You only need to request the storage resources you want, without being exposed to the details of how they are implemented.

  • A PV describes a persistent data storage volume. It defines a directory for persistent storage on a host machine, for example, a mount directory of a network file system (NFS).
  • A PVC describes the attributes of the PV that a pod wants to use, such as the volume capacity and read/write permissions.

To allow a pod to use a PV, the Kubernetes cluster administrator needs to configure a network StorageClass and provides PV descriptors to Kubernetes. You only need to create a PVC and bind it with the volumes in the pod so that you can store data. The following figure shows the interaction between a PV and PVC.

Figure 1 Interaction between a PV and PVC

CSI

Kubernetes Container Storage Interface (CSI) can be used to develop plug-ins to support specific storage volumes. For example, there are everest-csi-controller and everest-csi-driver developed by CCE in the kube-system namespace in Namespace for Grouping Resources. With these drivers, you can use cloud storage services such as EVS, SFS, and OBS.

$ kubectl get po --namespace=kube-system
NAME                                      READY   STATUS    RESTARTS   AGE
everest-csi-controller-6d796fb9c5-v22df   2/2     Running   0          9m11s
everest-csi-driver-snzrr                  1/1     Running   0          12m
everest-csi-driver-ttj28                  1/1     Running   0          12m
everest-csi-driver-wtrk6                  1/1     Running   0          12m

PV

Each PV contains the specification and status of the volume. For example, a file system is created in SFS, with the file system ID 68e4a4fd-d759-444b-8265-20dc66c8c502 and the mount point sfs-nas01.cn-north-4b.myhuaweicloud.com:/share-96314776. To use this file system in CCE, create a PV to describe the volume, as shown in the following example:

apiVersion: v1
kind: PersistentVolume
metadata:
  name: pv-example
spec:
  accessModes:
  - ReadWriteMany                      # Read/write mode
  capacity:
    storage: 10Gi                      # PV capacity
  csi:
    driver: nas.csi.everest.io         # Driver to be used
    fsType: nfs                        # StorageClass
    volumeAttributes:
      everest.io/share-export-location: sfs-nas01.cn-north-4b.myhuaweicloud.com:/share-96314776   # Mount point
    volumeHandle: 68e4a4fd-d759-444b-8265-20dc66c8c502                                            # Storage ID

Fields under csi in this example are dedicated used in CCE.

Next, create the PV and view its details.

$ kubectl create -f pv.yaml
persistentvolume/pv-example created

$ kubectl get pv
NAME                 CAPACITY   ACCESS MODES   RECLAIM POLICY   STATUS      CLAIM    STORAGECLASS   REASON   AGE
pv-example           10Gi       RWX            Retain           Available                                    4s

For RECLAIM POLICY, the value Retain indicates that the PV is retained after the PVC is released.

PVC

Each PVC can only be bound to one PV. The following is an example:

apiVersion: v1
kind: PersistentVolumeClaim
metadata:
  name: pvc-example
spec:
  accessModes:
  - ReadWriteMany
  resources:
    requests:
      storage: 10Gi              # Storage capacity
  volumeName: pv-example         # PV name

Create the PVC and view its details.

$ kubectl create -f pvc.yaml
persistentvolumeclaim/pvc-example created

$ kubectl get pvc
NAME          STATUS   VOLUME       CAPACITY   ACCESS MODES   STORAGECLASS   AGE
pvc-example   Bound    pv-example   10Gi       RWX                           9s

The command output shows that the PVC is in the Bound state and the value of VOLUME is pv-example, indicating that the PVC has been bound to a PV.

Then, check the PV status.

$ kubectl get pv
NAME          CAPACITY   ACCESS MODES   RECLAIM POLICY   STATUS   CLAIM                  STORAGECLASS   REASON   AGE
pv-example    10Gi       RWX            Retain           Bound    default/pvc-example                            50s

The status of the PV is also Bound. The value of CLAIM is default/pvc-example, indicating that the PV is bound to the PVC named pvc-example in the default namespace.

Note that PVs are cluster-level resources and do not belong to any namespace, while PVCs are namespace-level resources. PVs can be bound to PVCs of any namespace. Therefore, the namespace name default followed by the PVC name is displayed under CLAIM in this example.

Figure 2 Relationship between PVs and PVCs

StorageClass

PVs and PVCs allow you to consume storage resources, but creating them is a complex process, especially the csi field in PVs. In addition, PVs and PVCs are generally managed by the cluster administrator. It is inconvenient for you to configure varying attributes for them.

To solve this problem, Kubernetes supports dynamic PV provisioning to create PVs automatically. The cluster administrator can deploy a PV provisioner and define StorageClasses. In this way, you can select a desired StorageClass when creating a PVC. The PVC then transfers the StorageClass to the PV provisioner, and the provisioner automatically creates a PV. In CCE, StorageClasses such as csi-disk, csi-nas, and csi-obs are supported. The storageClassName field is added to a PVC so that PVs can be automatically provisioned and underlying storage resources can be automatically created.

Run the following command to obtain the StorageClasses that CCE supports. You can use the CSI add-ons provided by CCE to customize StorageClasses, which function similarly as the default StorageClasses in CCE.

# kubectl get sc
NAME                PROVISIONER                     AGE
csi-disk            everest-csi-provisioner         17d          # StorageClass for EVS disks
csi-disk-topology   everest-csi-provisioner         17d          # StorageClass for EVS disks with delayed association
csi-nas             everest-csi-provisioner         17d          # StorageClass for SFS file systems
csi-obs             everest-csi-provisioner         17d          # StorageClass for OBS buckets
csi-sfsturbo        everest-csi-provisioner         17d          # StorageClass for SFS Turbo file systems

Run the following command to specify a StorageClass for creating a PVC:

apiVersion: v1
kind: PersistentVolumeClaim
metadata:
  name:  pvc-sfs-auto-example
spec:
  accessModes:
  - ReadWriteMany
  resources:
    requests:
      storage: 10Gi
  storageClassName: csi-nas        # StorageClass

PVCs cannot be directly created by using the StorageClass csi-sfsturbo. To use SFS Turbo storage, create an SFS Turbo file system and then a PV and PVC through a static PV. For details, see Using an Existing SFS Turbo File System Through a Static PV.

Run the following command to create the PVC and view the PVC and PV details:

$ kubectl create -f pvc2.yaml
persistentvolumeclaim/pvc-sfs-auto-example created

$ kubectl get pvc
NAME                   STATUS   VOLUME                                     CAPACITY   ACCESS MODES   STORAGECLASS   AGE
pvc-sfs-auto-example   Bound    pvc-1f1c1812-f85f-41a6-a3b4-785d21063ff3   10Gi       RWX            csi-nas        29s

$ kubectl get pv
NAME                                       CAPACITY   ACCESS MODES   RECLAIM POLICY   STATUS   CLAIM                         STORAGECLASS   REASON   AGE
pvc-1f1c1812-f85f-41a6-a3b4-785d21063ff3   10Gi       RWO            Delete           Bound    default/pvc-sfs-auto-example  csi-nas                 20s

The command output shows that after a StorageClass is specified, a PVC and a PV are created and bound.

After a StorageClass is specified, PVs can be automatically created and maintained. You only need to specify StorageClassName when creating a PVC, which greatly reduces the workload.

Using a PVC in a Pod

You can directly bind an available PVC to a volume in the pod template and then mount the volume to the pod, as shown in the following example. You can also directly create a PVC in a StatefulSet. For details, see StatefulSets.

apiVersion: apps/v1
kind: Deployment
metadata:
  name: nginx-deployment
spec:
  selector:
    matchLabels:
      app: nginx
  replicas: 2
  template:
    metadata:
      labels:
        app: nginx
    spec:
      containers: 
      - image: nginx:alpine
        name: container-0 
        volumeMounts: 
        - mountPath: /tmp                                # Mount path
          name: pvc-sfs-example 
      restartPolicy: Always 
      volumes: 
      - name: pvc-sfs-example 
        persistentVolumeClaim: 
          claimName:  pvc-example                       # PVC name