Use FlexVolume to dynamically expand a disk volume in ACK - User Guide for Kubernetes Clusters| Alibaba Cloud Documentation Center

In Kubernetes 1.16, the feature to dynamically expand a disk volume is in public preview. Container Service for Kubernetes (ACK) allows you to dynamically expand a disk volume by using FlexVolume in Kubernetes 1.16 and later. This topic describes how to dynamically expand a disk volume by using FlexVolume.

Background information

You can expand a PV that contains a disk or file system without the need to unmount the PV from the directory. However, we recommend that you stop the application and unmount the PV from the directory for best practice if the PV contains a file system.

To ensure stability, ACK allows you to use the following methods to expand a PV:

Expand a volume without the need to restart the pod to which the volume is mounted. If the I/O throughput of the cluster is high when you use this method, a file system error may occur.
Restart the pod to which the PV is mounted and then expand the PV. This method ensures higher security because the PV is expanded after the application is stopped.

Kubernetes 1.16 and later allow you to expand a PV without the need to restart the pod to which the PV is mounted.

Usage notes

Limits
You can dynamically expand only disk volumes that are smaller than 2,000 GiB.
Data backup
To avoid data loss caused by volume expansion errors, make sure that you have created a snapshot for the PV.
Scenarios
- You can dynamically expand only volumes that are dynamically provisioned. Dynamically provisioned volumes refer to persistent volumes (PVs) that contain the StorageClassName attribute.
- ACK does not allow you to expand inline disk volumes. Inline disk volumes refer to disk volumes that are not created by using PVs and persistent volume claims (PVCs).
- ACK does not allow you to dynamically expand volumes that contain basic disks.
- Specify AllowVolumeExpansion: True for the StorageClass. The AllowVolumeExpansion field is automatically set to true for StorageClasses that are declared by ACK. For StorageClasses that are manually declared, you must manually set the AllowVolumeExpansion field to true.
Plug-in version
Make sure that the FlexVolume or CSI plug-in is upgraded to the latest version.

Grant the ResizeDisk permission to the RAM role

Before you dynamically expand a disk volume, you must grant the ResizeDisk permission to the RAM role of the cluster. To grant the ResizeDisk permission to the RAM role of a cluster based on the type and plug-in of the cluster, perform the following steps:

A dedicated Kubernetes cluster that has the CSI plug-in installed

Log on to the ACK console.
In the left-side navigation pane, click Clusters.
On the Clusters page, find the cluster that you want to manage and click Details in the Actions column.
In the left-side navigation pane, click Cluster Information.
Click the Cluster Resources tab and click the hyperlink next to Master RAM Role.
In the RAM console, grant the ResizeDisk permission to the RAM role. For more information, see Modify a custom policy.

A managed Kubernetes cluster or a dedicated Kubernetes cluster that has the FlexVolume plug-in installed

Repeat the preceding four steps. On the management page of the cluster, click the Cluster Resources tab and then click the hyperlink next to Worker RAM Role.

Expand a disk volume without the need to restart the pod

Use kubectl to connect to an ACK cluster.

In this example, the pod that you want to manage is in the following state

Run the following command to query the information about the pod:

kubectl get pod

The following output is returned:

web-0         1/1     Running   0          42s

Run the following command to view the mounting details of the pod:

kubectl exec web-0 df /data

The following output is returned:

Filesystem     1K-blocks  Used Available Use% Mounted on
/dev/vdb        20511312 45080  20449848   1% /data

Run the following command to query the information about the persistent volume claim (PVC):

kubectl get pvc

The following output is returned:

NAME             STATUS   VOLUME                   CAPACITY   ACCESS MODES   STORAGECLASS              AGE
disk-ssd-web-0   Bound    d-wz9hpoifm43yn9zie6gl   20Gi       RWO            alicloud-disk-available   57s

Run the following command to query the information about the persistent volume (PV):

kubectl get pv

The following output is returned:

NAME                     CAPACITY   ACCESS MODES   RECLAIM POLICY   STATUS     CLAIM                    STORAGECLASS              REASON   AGE
d-wz9hpoifm43yn9zie6gl   20Gi       RWO            Delete           Bound      default/disk-ssd-web-0   alicloud-disk-available            65s

Make sure that the requirements that are described in the Usage notes section are met. After the requirements are met, run the following command to expand the volume:

kubectl patch pvc disk-ssd-web-0 -p '{"spec":{"resources":{"requests":{"storage":"30Gi"}}}}'

Wait one minute and then perform the following steps to check whether the volume is expanded.

Run the following command to query the information about the PV:

kubectl get pv d-wz9hpoifm43yn9zie6gl

The following output is returned:

NAME                     CAPACITY   ACCESS MODES   RECLAIM POLICY   STATUS   CLAIM                    STORAGECLASS              REASON   AGE
d-wz9hpoifm43yn9zie6gl   30Gi       RWO            Delete           Bound    default/disk-ssd-web-0   alicloud-disk-available            5m23s

Run the following command to query the information about the PVC:

kubectl get pvc

The following output is returned:

NAME             STATUS   VOLUME                   CAPACITY   ACCESS MODES   STORAGECLASS              AGE
disk-ssd-web-0   Bound    d-wz9hpoifm43yn9zie6gl   30Gi       RWO            alicloud-disk-available   5m10s

Run the following command to view the mounting details of the pod:

kubectl exec web-0 df /data

The following output is returned:

Filesystem     1K-blocks  Used Available Use% Mounted on
/dev/vdb        30832548 45036  30771128   1% /data

To expand a disk volume without the need to restart the pod, you need only to run the preceding command.

Restart the pod to which a disk volume is mounted and then expand the disk volume

Use kubectl to connect to a Kubernetes cluster. For more information, see Use kubectl to connect to an ACK cluster.

In this example, the pod that you want to manage is in the following state.

Run the following command to query the information about the pod:

kubectl get pod

The following output is returned:

web-0         1/1     Running   0          42s

Run the following command to view the mounting details of the pod:

kubectl exec web-0 df /data

The following output is returned:

Filesystem     1K-blocks  Used Available Use% Mounted on
/dev/vdb        20511312 45080  20449848   1% /data

Run the following command to query the information about the PVC:

kubectl get pvc

The following output is returned:

NAME             STATUS   VOLUME                   CAPACITY   ACCESS MODES   STORAGECLASS              AGE
disk-ssd-web-0   Bound    d-wz9hpoifm43yn9zie6gl   20Gi       RWO            alicloud-disk-available   57s

Run the following command to query the information about the PV:

kubectl get pv

The following output is returned:

NAME                     CAPACITY   ACCESS MODES   RECLAIM POLICY   STATUS     CLAIM                    STORAGECLASS              REASON   AGE
d-wz9hpoifm43yn9zie6gl   20Gi       RWO            Delete           Bound      default/disk-ssd-web-0   alicloud-disk-available            65s

Run the following command to view the scheduling information about the PV:

kubectl get pv d-wz9g2j5qbo37r2lamkg4  -oyaml | grep failure-domain.beta.kubernetes.io/zone
    failure-domain.beta.kubernetes.io/zone: cn-shenzhen-e

Replace the value of the zone field in the label of the scheduled resource. Then, the pod to which the PV is mounted cannot be scheduled.
For example, you can change the value of the zone field in the label from cn-shenzhen-e to cn-shenzhen-e-nozone.
```
kubectl label pv d-wz9g2j5qbo37r2lamkg4 failure-domain.beta.kubernetes.io/zone=cn-shenzhen-e-nozone --overwrite
persistentvolume/d-wz9g2j5qbo37r2lamkg4 labeled
```
Restart the pod.
The pod scheduling setting is modified. Therefore, the pod temporarily remains in the Pending state.

Run the following command to delete the pod:
```
kubectl delete pod web-0
```
Run the following command to query the information about the pod:
```
kubectl get pod
```
The following output is returned:
```
web-0   0/1     Pending   0          27s
```

Run the following command to expand the volume:

kubectl patch pvc disk-ssd-web-0 -p '{"spec":{"resources":{"requests":{"storage":"30Gi"}}}}'

Restart the pod by changing the zone field in the label to the previous setting. In this example, change the value from cn-shenzhen-e-nozone to cn-shenzhen-e.
```
kubectl label pv d-wz9g2j5qbo37r2lamkg4 failure-domain.beta.kubernetes.io/zone=cn-shenzhen-e --overwrite
persistentvolume/d-wz9g2j5qbo37r2lamkg4 labeled
```
Wait one minute and then perform the following steps to check whether the volume is expanded.
Run the following command to query the information about the pod:
```
kubectl get pod
```
The following output is returned:
```
web-0   1/1     Running   0          3m23s
```
Run the following command to query the information about the PVC:
```
kubectl get pvc
```
The following output is returned:
```
disk-ssd-web-0   Bound    d-wz9g2j5qbo37r2lamkg4   30Gi       RWO            alicloud-disk-available   17m
```
Run the following command to query the information about the PV:
```
kubectl get pv d-wz9g2j5qbo37r2lamkg4
```
The following output is returned:
```
d-wz9g2j5qbo37r2lamkg4   30Gi       RWO            Delete           Bound    default/disk-ssd-web-0   alicloud-disk-available            17m
```
Run the following command to view the mounting details of the pod:
```
kubectl exec web-0 df /data
```
The following output is returned:
```
/dev/vdb        30832548 45036  30771128   1% /data
```
The result indicates that the disk volume is expanded from 20 GiB to 30 GiB.