Use a dynamic cloud disk when creating a stateful service

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Last Updated: Jan 07, 2019 Edit in Github

This topic describes typical scenarios in which a dynamic cloud disk is needed for creating a stateful service, and the procedure for how to use one.

Scenarios and method

Scenario for using dynamic cloud disks:

You want to configure the system to automatically purchase cloud disks when you deploy an application, rather than manually purchase cloud disks before deploying the application.

Method of using a dynamic cloud disk:

Manually create a PVC and claim a specific StorageClass in the PVC.
Use the StorageClass to enable the system to automatically create a PV when you deploy an application.

Prerequisites

You have created a Kubernetes cluster. For more information, see Create a Kubernetes cluster.
You have connected to the Kubernetes cluster by using kubectl, see Connect to a Kubernetes cluster by using kubectl.
You have installed the provisioner plugin in the Kubernetes cluster. The plugin automatically creates a cloud disk according to a specific StorageClass.

Provisioner plugin

When you create a cluster through Alibaba Cloud Container Service for Kubernetes, the provisioner plugin is installed in the cluster by default.

Create a StorageClass

By default, Alibaba Cloud Container Service for Kubernetes creates four StorageClasses for a cluster during the cluster initialization, and the StorageClasses use the default settings. Furthermore, the four default StorageClasses are created only for a cluster that has a single zone. For a cluster that has multiple zones, you need to manually create a StorageClass. The following are the four StorageClasses created by default:

alicloud-disk-common indicates to automatically create a basic cloud disk.
alicloud-disk-efficiency indicates to automatically create an Ultra cloud disk.
alicloud-disk-ssd indicates to automatically create an SSD cloud disk.
alicloud-disk-available indicates a systematic method of disk selection. Specifically, the system first attempts to create an Ultra cloud disk. If the Ultra cloud disks in the specified zone are sold out, the system tries to create an SSD cloud disk. If the SSD cloud disks are sold out, the system tries to create a basic cloud disk.

Create a storageclass.yaml file.
```
kind: StorageClass
apiVersion: storage.k8s.io/v1beta1
metadata:
  name: alicloud-disk-ssd-hangzhou-b
provisioner: alicloud/disk
reclaimPolicy: Retain
parameters:
  type: cloud_ssd
  regionid: cn-hangzhou
  zoneid: cn-hangzhou-b
  fstype: "ext4"
  readonly: "false"
```
Parameter setting
- provisioner: Set this parameter to alicloud/disk to specify that the StorageClass creates an Alibaba Cloud cloud disk by using the provisioner plugin.
- reclaimPolicy: Set a policy to reclaim the cloud disk. Available values of this parameter are Delete and Retain. The default setting is Delete.
  
  Note If you maintain the default setting, namely, Delete, the data on the cloud disk cannot be restored after you remove the PVC because the cloud disk is also removed.
- type: Specify a cloud disk type by using one the following values: cloud, cloud_efficiency, cloud_ssd, and available.
- regionid: (optional) Set the region in which the cloud disk is automatically created. This region must be the same as the region in which your cluster resides.
- zoneid: (optional) Set the zone in which a cloud disk is automatically created.
  - If you set this parameter for a single-zone cluster, the value must be the same as the zone in which the cluster resides.
  - If you set this parameter for a multi-zone cluster, multiple values can be set. For example,
```
zoneid: cn-hangzhou-a,cn-hangzhou-b,cn-hangzhou-c
```
- fstype: (optional) Set the type of the file system used for automatic cloud disk creation. The default setting is ext4.
- readonly: (optional) Set whether the automatically created cloud disk is read only. If you set this parameter to true, the cloud disk can only be read. If you set this parameter to false, the cloud disk can be read and written. The default setting is false.
- encrypted: (optional) Set whether to encrypt the automatically created cloud disk. If you set this parameter to true, the cloud disk is encrypted. If you set this parameter to false, the cloud disk is not encrypted. The default setting is false.
Run the following command to create a StorageClass:
```
$ kubectl create -f storageclass.yaml
```

Create a PVC

Create a pvc-ssd.yaml file.

kind: PersistentVolumeClaim
apiVersion: v1
metadata:
  name: disk-ssd
spec:
  accessModes:
    - ReadWriteOnce
  storageClassName: alicloud-disk-ssd-hangzhou-b
  resources:
    requests:
      storage: 20Gi

Run the following command to create a PVC:
```
$ kubectl create -f pvc-ssd.yaml
```

Result

In the left-side navigation pane under Kubernetes, choose Application > Volumes Claim, and select the target cluster and namespace to see that the storage class name associated to the PVC is alicloud-disk-ssd-hangzhou-b specified in the StorageClass, and the PVC is associated with the volume.

Create an application

Create a pvc-dynamic.yaml file.

apiVersion: apps/v1
kind: Deployment
metadata:
  name: nginx-dynamic
  labels:
    app: nginx
spec:
  selector:
    matchLabels:
      app: nginx
  template:
    metadata:
      labels:
        app: nginx
    spec:
      containers:
      - name: nginx
        image: nginx
        volumeMounts:
          - name: disk-pvc
            mountPath: "/data"
      volumes:
        - name: disk-pvc
          persistentVolumeClaim:
            claimName: disk-ssd

Run the following command to create a deployment:
```
$ kubectl create -f nginx-dynamic.yaml
```

Result

In the left-side navigation pane under Kubernetes, choose Application > Deployment, and select the target cluster and namespace to see the created deployment.

Persistent storage for a dynamic cloud disk

Run the following command to view the pod in which the created deployment resides:

$ kubectl get pod | grep dynamic
nginx-dynamic-5c74594ccb-zl9pf     2/2     Running     0          3m

Run the following command to check whether a new cloud disk is mounted to the /data path:

$ kubectl exec nginx-dynamic-5c74594ccb-zl9pf df | grep data
/dev/vdh        20511312    45080  20449848   1% /data

Run the following command to view the file in the /data path:

$ kubectl exec nginx-dynamic-5c74594ccb-zl9pf ls /data
lost+found

Run the following command to create a file named dynamic in the /data path:
```
$ kubectl exec nginx-dynamic-5c74594ccb-zl9pf touch /data/dynamic
```

Run the following command to view the files in the /data path:

$ kubectl exec nginx-dynamic-5c74594ccb-zl9pf ls /data
dynamic
lost+found

Run the following command to remove the pod named nginx-dynamic-78c7dcb9d7-g9lll:

$ kubectl delete pod nginx-dynamic-5c74594ccb-zl9pf
pod "nginx-dynamic-5c74594ccb-zl9pf" deleted

Open another kubectl interface and run the following command to view the process in which the preceding pod is removed and a new pod is created by Kubernetes:

$ kubectl get pod -w -l app=nginx
NAME                               READY   STATUS    RESTARTS   AGE
nginx-dynamic-5c74594ccb-zl9pf     2/2     Running   0          6m48s
nginx-dynamic-5c74594ccb-zl9pf   2/2   Terminating   0     7m32s
nginx-dynamic-5c74594ccb-45sd4   0/2   Pending   0     0s
nginx-dynamic-5c74594ccb-45sd4   0/2   Pending   0     0s
nginx-dynamic-5c74594ccb-45sd4   0/2   Init:0/1   0     0s
nginx-dynamic-5c74594ccb-zl9pf   0/2   Terminating   0     7m32s
nginx-dynamic-5c74594ccb-zl9pf   0/2   Terminating   0     7m33s
nginx-dynamic-5c74594ccb-zl9pf   0/2   Terminating   0     7m33s
nginx-dynamic-5c74594ccb-45sd4   0/2   PodInitializing   0     5s
nginx-dynamic-5c74594ccb-45sd4   2/2   Running   0     22s

Run the following command to view the pod newly created by Kubernetes:

$ kubectl get pod 
NAME                               READY   STATUS      RESTARTS   AGE
nginx-dynamic-5c74594ccb-45sd4     2/2     Running     0          2m

Run the following command to verify that the created file named dynamic in the /data path has not been removed, indicating that data in the dynamic cloud disk can be stored persistently:
```
$ kubectl exec nginx-dynamic-5c74594ccb-45sd4 ls /data
dynamic
lost+found
```