You can create a pair of persistent volume (PV) and persistent volume claim (PVC) for each pod by setting the VolumeClaimTemplate in a StatefulSet. If pods are deleted or scaled in, PVs and PVCs of the StatefulSet application are not deleted. This topic describes how to enable a StatefulSet to support persistent storage by setting the VolumeClaimTemplate.
Prerequisites
Create an ACK managed clusterScenarios
- Predefined deployment order: Pods are deployed or scaled out from 0 to N-1 in sequence. The system must wait until all of the preceding pods reach the Running or Ready state before it can deploy another pod.
- Predefined scale-in order: Pods are scaled in or deleted from N-1 to 0 in sequence. The system must wait until all of the preceding pods are deleted before it can delete another pod.
- Consistent network identifiers: After a pod is rescheduled, its PodName and HostName values remain unchanged.
- Stable data persistence: After a pod is rescheduled, the pod can still access the same persisted data.
Create a StatefulSet
PVCs and PVs can be automatically created based on VolumeClaimTemplates
.
- Use the following template to create a file named statefulset.yaml. Deploy a Service and a StatefulSet, and provision two pods for the StatefulSet.
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: 2 template: metadata: labels: app: nginx spec: containers: - name: nginx image: nginx ports: - containerPort: 80 name: web volumeMounts: - name: disk-ssd mountPath: /data volumeClaimTemplates: - metadata: name: disk-ssd spec: accessModes: [ "ReadWriteOnce" ] storageClassName: "alicloud-disk-ssd" resources: requests: storage: 20Gi
replicas
: the parameter is set to 2 in this example. This indicates that two pods are created.mountPath
: the path where you want to mount the disk in the container.accessModes
: the access mode of the StatefulSet.storageClassName
: the parameter is set toalicloud-disk-ssd
in this example. This indicates that an Alibaba Cloud standard SSD is used.storage
: specifies the storage that is required by the application.
- Run the following command to create a StatefulSet:
kubectl create -f statefulset.yaml
- Run the following command to query the deployed pods:
kubectl get pod
Expected output:
NAME READY STATUS RESTARTS AGE web-0 1/1 Running 0 6m web-1 1/1 Running 0 6m
- Run the following command to query the PVCs:
kubectl get pvc
Expected output:
NAME STATUS VOLUME CAPACITY ACCESS MODES STORAGECLASS AGE disk-ssd-web-0 Bound d-2zegw7et6xc96nbojuoo 20Gi RWO alicloud-disk-ssd 7m disk-ssd-web-1 Bound d-2zefbrqggvkd10xb523h 20Gi RWO alicloud-disk-ssd 6m
Verify that the PVCs are scaled out together with the StatefulSet application
- Run the following command to scale out the StatefulSet application to three pods:
kubectl scale sts web --replicas=3
Expected output:
statefulset.apps/web scaled
- Run the following command to view the pods after the StatefulSet application is scaled out:
kubectl get pod
Expected output:
NAME READY STATUS RESTARTS AGE web-0 1/1 Running 0 34m web-1 1/1 Running 0 33m web-2 1/1 Running 0 26m
- Run the following command to view the PVCs after the StatefulSet application is scaled out:
kubectl get pvc
Expected output:
NAME STATUS VOLUME CAPACITY ACCESS MODES STORAGECLASS AGE disk-ssd-web-0 Bound d-2zegw7et6xc96nbojuoo 20Gi RWO alicloud-disk-ssd 35m disk-ssd-web-1 Bound d-2zefbrqggvkd10xb523h 20Gi RWO alicloud-disk-ssd 34m disk-ssd-web-2 Bound d-2ze4jx1zymn4n9j3pic2 20Gi RWO alicloud-disk-ssd 27m
The output indicates that three PVCs are provisioned for the StatefulSet application after the StatefulSet application is scaled to three pods.
Verify that the PVCs remain unchanged after the StatefulSet application is scaled in.
- Run the following command to scale the StatefulSet application to two pods:
kubectl scale sts web --replicas=2
Expected output:
statefulset.apps/web scaled
- Run the following command to view the pods after the StatefulSet application is scaled in:
kubectl get pod
Expected output:
NAME READY STATUS RESTARTS AGE web-0 1/1 Running 0 38m web-1 1/1 Running 0 38m
Only two pods are deployed for the StatefulSet application. - Run the following command to view the PVCs after the StatefulSet application is scaled in:
kubectl get pvc
Expected output:
NAME STATUS VOLUME CAPACITY ACCESS MODES STORAGECLASS AGE disk-ssd-web-0 Bound d-2zegw7et6xc96nbojuoo 20Gi RWO alicloud-disk-ssd 39m disk-ssd-web-1 Bound d-2zefbrqggvkd10xb523h 20Gi RWO alicloud-disk-ssd 39m disk-ssd-web-2 Bound d-2ze4jx1zymn4n9j3pic2 20Gi RWO alicloud-disk-ssd 31m
After the StatefulSet application is scaled to two pods, the StatefulSet application still has three PVCs. This indicates that the PVCs are not scaled in together with the StatefulSet application.
Verify that the PVCs remain unchanged when the StatefulSet application is scaled out again
When the StatefulSet is scaled out again, verify that the PVCs remain unchanged.
- Run the following command to scale out the StatefulSet application to three pods:
kubectl scale sts web --replicas=3
Expected output:
statefulset.apps/web scaled
- Run the following command to view the pods after the StatefulSet application is scaled out:
kubectl get pod
Expected output:
NAME READY STATUS RESTARTS AGE web-0 1/1 Running 0 1h web-1 1/1 Running 0 1h web-2 1/1 Running 0 8s
- Run the following command to view the PVCs after the StatefulSet application is scaled out:
kubectl get pvc
Expected output:
NAME STATUS VOLUME CAPACITY ACCESS MODES STORAGECLASS AGE disk-ssd-web-0 Bound d-2zegw7et6xc96nbojuoo 20Gi RWO alicloud-disk-ssd 1h disk-ssd-web-1 Bound d-2zefbrqggvkd10xb523h 20Gi RWO alicloud-disk-ssd 1h disk-ssd-web-2 Bound d-2ze4jx1zymn4n9j3pic2 20Gi RWO alicloud-disk-ssd 1h
The newly created pod uses an existing PVC.
Verify that the PVCs remain unchanged when the pod of the StatefulSet application is deleted
- Run the following command to view the PVC that is used by the pod named web-1:
kubectl describe pod web-1 | grep ClaimName
Expected output:
ClaimName: disk-ssd-web-1
- Run the following command to delete the pod named web-1:
kubectl delete pod web-1
Expected output:
pod "web-1" deleted
- Run the following command to view the pod:
kubectl get pod
Expected output:
NAME READY STATUS RESTARTS AGE web-0 1/1 Running 0 1h web-1 1/1 Running 0 25s web-2 1/1 Running 0 9m
The recreated pod uses the same name as the deleted pod. - Run the following command to view the PVCs:
kubectl get pvc
Expected output:
NAME STATUS VOLUME CAPACITY ACCESS MODES STORAGECLASS AGE disk-ssd-web-0 Bound d-2zegw7et6xc96nbojuoo 20Gi RWO alicloud-disk-ssd 1h disk-ssd-web-1 Bound d-2zefbrqggvkd10xb523h 20Gi RWO alicloud-disk-ssd 1h disk-ssd-web-2 Bound d-2ze4jx1zymn4n9j3pic2 20Gi RWO alicloud-disk-ssd 1h
The recreated pod uses an existing PVC.
Verify that the StatefulSet application supports persistent storage
- Run the following command to view the file in the /data path:
kubectl exec web-1 -- ls /data
Expected output:
lost+found
- Run the following command to create a file named statefulset in the /data path:
kubectl exec web-1 -- touch /data/statefulset
- Run the following command to view the file in the /data path:
kubectl exec web-1 -- ls /data
Expected output:
lost+found statefulset
- Run the following command to delete the pod named web-1:
kubectl delete pod web-1
Expected output:
pod "web-1" deleted
- Run the following command to view the file in the /data path:
kubectl exec web-1 -- ls /data
Expected output:
lost+found statefulset
The statefulset file still exists in the /data path. This indicates that data is persisted to the disk.