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

Prerequisites

The RAM role of the cluster that you want to manage is granted the ResizeDisk permission. The procedure of granting the ResizeDisk permission to a RAM role varies based on the cluster type and the plug-in type.
  • A dedicated cluster that has the CSI plug-in:
    1. Log on to the ACK console.
    2. In the left-side navigation pane, click Clusters.
    3. On the Clusters page, find the cluster that you want to manage and click Details in the Actions column.
    4. In the left-side navigation pane, click Cluster Information.
    5. Click the Cluster Resources tab and click the link next to Master RAM Role.
    6. In the RAM console, grant the ResizeDisk permission to the RAM role. For more information, see Modify a custom policy.resizedisk
  • A managed cluster or a dedicated cluster that has the FlexVolume plug-in:

    Perform the first four steps of the preceding procedure. On the management page of the cluster, click the Cluster Resources tab and click the link next to Worker RAM Role.

Background information

PV expansion consists of the manual expansion of PVs and the automatic expansion of file systems. ACK allows you to expand both PVs and file systems without unmounting PVs from the host directory. However, to ensure the stability of file system expansion, stop application services and unmount the PV from the host directory first.

To meet different levels of stability requirements, ACK provides the following solutions for PV expansion:
  • Expand a PV without restarting the pod to which the PV is mounted. If you use this method, file system errors may occur if the cluster I/O is high.
  • Expand a PV during the restart of the pod to which the PV is mounted. To ensure data security during expansion, stop application services before expansion.

By default, ACK V1.16 and later support PV expansion without restarting the pod.

Notes

  • Data backup

    To avoid data loss caused by PV expansion errors, make sure that snapshots are created for the PV.

  • Scenarios
    • You can enable dynamic expansion for a PV only if the StorageClassName parameter is specified for the PV.
    • ACK does not support expansion of inline volumes. Only PVs can be expanded.
    • ACK does not support dynamic expansion of basic disks.
    • The AllowVolumeExpansion field of a StorageClass is set to true. If you create a StorageClass, you must set this field to true.
  • Plug-in version

    Make sure that the FlexVolume or CSI plug-in is upgraded to the latest version.

Expand a disk type PV without restarting the pod

  1. Use kubectl to connect to an ACK cluster.

    Assume that the pod that you want to manage is in the following state.

    Run the following command to query 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 state 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 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 information about the PV that is related to the PVC:

    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
  2. Make sure that the requirements described in theNotes section are met. Then, run the following command to expand the PV:
    kubectl patch pvc disk-ssd-web-0 -p '{"spec":{"resources":{"requests":{"storage":"30Gi"}}}}'
    Wait one minute and perform the following steps to check whether the PV is expanded.

    Run the following command to query 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 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 state 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 restarting a pod, you only need to run the preceding command to complete all the required operations.

Expand a disk type PV after restarting a pod

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

    Assume that 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 command output appears:

    web-0         1/1     Running   0          42s

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

    kubectl exec web-0 df /data

    The following command output appears:

    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 command output appears:
    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 command output appears:
    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
  2. 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
  3. Modify the label of the scheduled resource by replacing the value of the zone field. 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
  4. Restart the pod.

    The pod temporarily remains in the Pending state because the zone field of the label is modified.

    Run the following command to delete the pod:

    kubectl delete pod web-0

    Run the following command to query information about the pod:

    kubectl get pod

    The following output is returned:

    web-0   0/1     Pending   0          27s
  5. Run the following command to expand the PV:
    kubectl patch pvc disk-ssd-web-0 -p '{"spec":{"resources":{"requests":{"storage":"30Gi"}}}}'
  6. Change the zone field in the label to the previous setting to restart the pod. 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 perform the following steps to check whether the PV is expanded.

    Run the following command to query 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 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 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 status of the pod:

    kubectl exec web-0 df /data

    The following output is returned:

    /dev/vdb        30832548 45036  30771128   1% /data

    The result shows that the PV is expanded from 20 GiB to 30 GiB.