Dynamically Mounting a Local PV to a StatefulSet

Application Scenarios

Dynamic mounting is available only for creating a StatefulSet. It is implemented through a volume claim template (volumeClaimTemplates field) and depends on dynamic creation of PVs through StorageClass. In this mode, each pod in a multi-pod StatefulSet is associated with a unique PVC and PV. After a pod is rescheduled, the original data can still be mounted to it based on the PVC name. In the common mounting mode for a Deployment, if ReadWriteMany is supported, multiple pods of the Deployment will be mounted to the same underlying storage.

When updating a StatefulSet in Kubernetes, it is not allowed to add or delete the volumeClaimTemplates field. This field can only be configured during the creation of the StatefulSet.

Prerequisites

• You have created a cluster and installed the CSI add-on (Everest) in the cluster.
• To create a cluster using commands, ensure kubectl is used. For details, see Connecting to a Cluster Using kubectl.
• You have imported a data disk of a node to the local PV storage pool. For details, see Importing a PV to a Storage Pool.

Dynamically Mounting a Local PV on the Console

1. Log in to the CCE console and click the cluster name to access the cluster console.
2. Choose Workloads in the navigation pane. In the right pane, click the StatefulSets tab.
3. Click Create Workload in the upper right corner. On the displayed page, click Data Storage in the Container Settings area and click Add Volume to select VolumeClaimTemplate.
4. Click Create PVC. In the dialog box displayed, configure the volume claim template parameters.
   Click Create.

   Parameter	Description
   PVC Type	In this section, select Local PV.
   PVC Name	Enter the name of the PVC. After a PVC is created, a suffix is automatically added based on the number of pods. The format is <Custom PVC name>-<Serial number>, for example, example-0.
   Creation Method	You can only select Dynamically provision to create a PVC, PV, and underlying storage on the console in cascading mode.
   Storage Classes	The default StorageClass of local PVs is csi-local-topology.
   Capacity	Requested capacity of the storage volume in GiB or MiB. NOTE: To implement local PVs, Logical Volume Manager (LVM) is used with a local extent (LE) of 4 MiB. If the requested PVC capacity is not a multiple of 4 MiB, the LVM logical volume's actual capacity will be rounded up to the nearest multiple of 4 MiB. For example, if you request 401 MiB of PVC capacity, the LVM logical volume's actual capacity will be 404 MiB (101 LEs), which is the same as what is displayed on the page.
   Access Mode	Local PVs support only ReadWriteOnce, indicating that a PV can be mounted to one node in read/write mode. For details, see Volume Access Modes.
   Storage Pool	View the imported storage pool. For details about how to import a new data volume to the storage pool, see Importing a PV to a Storage Pool.

5. Enter the path to which the volume is mounted.

   Table 1 Mounting a storage volume

   Parameter	Description
   Mount Path	Enter a mount path, for example, /tmp. This parameter specifies a container path to which a data volume will be mounted. Do not mount the volume to a system directory such as / or /var/run. This may lead to container errors. Mount the volume to an empty directory. If the directory is not empty, ensure that there are no files that affect container startup. Otherwise, the files will be replaced, leading to container startup failures or workload creation failures. NOTICE: If a volume is mounted to a high-risk directory, use an account with minimum permissions to start the container. Otherwise, high-risk files on the host may be damaged.
   Subpath	Enter the subpath of the storage volume and mount a path in the storage volume to the container. In this way, different folders of the same storage volume can be used in a single pod. tmp, for example, indicates that data in the mount path of the container is stored in the tmp folder of the storage volume. If this parameter is left blank, the root path will be used by default.
   Permission	Read-only: You can only read the data in the mounted volumes. Read-write: You can modify the data volumes mounted to the path. Newly written data will not be migrated if the container is migrated, which may cause data loss.

   In this example, the disk is mounted to the /data path of the container. The container data generated in this path is stored in the local PV.

   

6. Dynamically mount and use storage volumes. For details about other parameters, see Creating a StatefulSet. After the configuration, click Create Workload.

   After the workload is created, the data in the container mount directory will be persistently stored. Verify the storage by referring to Verifying Data Persistence.

Dynamically Mounting a Local PV Through kubectl

1. Use kubectl to access the cluster.
2. Create a file named statefulset-local.yaml. In this example, the local PV is mounted to the /data path.

   apiVersion: apps/v1
   kind: StatefulSet
   metadata:
     name: statefulset-local
     namespace: default
   spec:
     selector:
       matchLabels:
         app: statefulset-local
     template:
       metadata:
         labels:
           app: statefulset-local
       spec:
         containers:
           - name: container-1
             image: nginx:latest
             volumeMounts:
               - name: pvc-local          # The value must be the same as that in the volumeClaimTemplates field.
                 mountPath: /data         # Location where the storage volume is mounted
         imagePullSecrets:
           - name: default-secret
     serviceName: statefulset-local       # Headless Service name
     replicas: 2
     volumeClaimTemplates:
       - apiVersion: v1
         kind: PersistentVolumeClaim
         metadata:
           name: pvc-local
           namespace: default
   
   
         spec:
           accessModes:
             - ReadWriteOnce               # The value must be ReadWriteOnce for local PVs.
           resources:
             requests:
               storage: 10Gi               # Storage volume capacity
           storageClassName: csi-local-topology      # StorageClass is local PV.
   ---
   apiVersion: v1
   kind: Service
   metadata:
     name: statefulset-local   # Headless Service name
     namespace: default
     labels:
       app: statefulset-local
   spec:
     selector:
       app: statefulset-local
     clusterIP: None
     ports:
       - name: statefulset-local
         targetPort: 80
         nodePort: 0
         port: 80
         protocol: TCP
     type: ClusterIP

   Table 2 Key parameters

   Parameter	Mandatory	Description
   storage	Yes	Requested PVC capacity in Gi or Mi. NOTE: To implement local PVs, Logical Volume Manager (LVM) is used with a local extent (LE) of 4 MiB. If the requested PVC capacity is not a multiple of 4 MiB, the LVM logical volume's actual capacity will be rounded up to the nearest multiple of 4 MiB. For example, if you request 401 MiB of PVC capacity, the LVM logical volume's actual capacity will be 404 MiB (101 LEs), which is the same as what is displayed on the page.
   storageClassName	Yes	The StorageClass of local PVs is csi-local-topology.

3. Run the following command to create a workload to which the local PV is mounted:

   kubectl apply -f statefulset-local.yaml

   After the workload is created, you can try Verifying Data Persistence.

Verifying Data Persistence

1. View the deployed application and files.
   1. Run the following command to view the created pod:

      kubectl get pod | grep statefulset-local

      Expected output:

      statefulset-local-0          1/1     Running   0             45s
      statefulset-local-1          1/1     Running   0             28s

   2. Run the following command to check whether the local PV has been mounted to the /data path:

      kubectl exec statefulset-local-0 -- df | grep data

      Expected output:

      /dev/mapper/vg--everest--localvolume--persistent-pvc-local              10255636     36888  10202364   0% /data

   3. Run the following command to check the files in the /data path:

      kubectl exec statefulset-local-0 -- ls /data

      Expected output:

      lost+found

2. Run the following command to create a file named static in the /data path:

   kubectl exec statefulset-local-0 --  touch /data/static

3. Run the following command to check the files in the /data path:

   kubectl exec statefulset-local-0 -- ls /data

   Expected output:

   lost+found
   static

4. Run the following command to delete the pod named web-local-auto-0:

   kubectl delete pod statefulset-local-0

   Expected output:

   pod "statefulset-local-0" deleted

5. After the deletion, the StatefulSet controller automatically creates a replica with the same name. Run the following command to check whether the files in the /data path have been modified:

   kubectl exec statefulset-local-0 -- ls /data

   Expected output:

   lost+found
   static

   The static file is retained, indicating that the data in the local PV can be stored persistently.

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