PersistentVolumes, PersistentVolumeClaims, and StorageClasses
hostPath volumes are used for persistent storage. However, hostPath volumes are node-specific. Writing data into hostPath volumes after a node restart may cause data inconsistency.
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 storage, file storage, 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 PVs, a Kubernetes cluster administrator needs to set the network storage class and provides the corresponding PV descriptors to Kubernetes. You only need to create a PVC and bind the PVC with the volumes in the pod so that you can store data. The following figure shows the relationship between PVs and PVCs.
CSI
Kubernetes Container Storage Interface (CSI) can be used to develop plug-ins to support specific storage volumes. For example, in the namespace named kube-system, as shown in Namespaces: Grouping Resources, everest-csi-controller-* and everest-csi-driver-* are the storage controllers and drivers developed by CCE. 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. The file system ID is 68e4a4fd-d759-444b-8265-20dc66c8c502, and the mount point is sfs-nas01.eu-west-0a.prod-cloud-ocb.orange-business.com:/share-96314776. If you want to use this file system in CCE, you need to create a PV to describe the volume. The following is an example PV.
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 # File system type
volumeAttributes:
everest.io/share-export-location: sfs-nas01.eu-west-0a.prod-cloud-ocb.orange-business.com:/share-96314776 # Mount point
volumeHandle: 68e4a4fd-d759-444b-8265-20dc66c8c502 # File system ID
Fields under csi in the example above are exclusively used in CCE.
Next, create the PV.
$ 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
RECLAIM POLICY indicates the PV reclaim policy. The value Retain indicates that the PV is retained after the PVC is released. If the value of STATUS is Available, the PV is available.
PVC
A PVC can be bound to a 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.
$ 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 Bound state and the value of VOLUME is pv-example, indicating that the PVC is bound to a PV.
Now 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 PVC 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" is displayed under CLAIM.
StorageClass
Although PVs and PVCs allow you to consume abstract storage resources, you may need to configure multiple fields to create PVs and PVCs (such as the csi field structure in the PV), and PVs/PVCs are generally managed by the cluster administrator, which can be inconvenient when you need PVs with varying attributes for different problems.
To solve this problem, Kubernetes supports dynamic PV provisioning to create PVs automatically. The cluster administrator can deploy a PV provisioner and define the corresponding StorageClass. In this way, developers can select the storage class to be created when creating a PVC. The PVC transfers the StorageClass to the PV provisioner, and the provisioner automatically creates a PV. In CCE, storage classes such as csi-disk, csi-nas, and csi-obs are supported. After StorageClassName is added to a PVC, PVs can be automatically provisioned and underlying storage resources can be automatically created.
The following examples are for CCE clusters of v1.15 and later, different from those for CCE clusters of v1.13 and earlier.
Run the following command to query the storage classes that CCE supports. You can use the CSI plug-ins provided by CCE to customize a storage class, which functions similarly as the default storage classes in CCE.
# kubectl get sc NAME PROVISIONER AGE csi-disk everest-csi-provisioner 17d # Storage class for EVS disks csi-disk-topology everest-csi-provisioner 17d # Storage class for EVS disks with delayed association csi-nas everest-csi-provisioner 17d # Storage class for SFS file systems csi-obs everest-csi-provisioner 17d # Storage class for OBS buckets csi-sfsturbo everest-csi-provisioner 17d # Storage class for SFS Turbo file systems
Use storageClassName to create a PVC.
apiVersion: v1
kind: PersistentVolumeClaim
metadata:
name: pvc-sfs-auto-example
spec:
accessModes:
- ReadWriteMany
resources:
requests:
storage: 10Gi
storageClassName: csi-nas # StorageClass
Create a 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 used, a PVC and a PV are created and they are bound to each other.
After a StorageClass is set, PVs can be automatically created and maintained. Users only need to specify StorageClassName when creating a PVC, which greatly reduces the workload.
Note that the types of storageClassName vary among vendors. In this section, SFS is used as an example. For details about other storage classes, see Storage Overview.
Using a PVC in a Pod
After a PVC is available, you can directly bind the 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 StatefulSet.
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
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