Managing Virtual Machines with Red Hat OpenShift Virtualization

The source section of a data volume resource provides the details of the disk image to inject into the PV. The following screen capture shows that you can select the source from the Add disk window when using the OpenShift web console.

Figure 5.4: Select the data source

You can detach disks only from stopped VMs. After you stop the VM, navigate to Configuration → Disks, click the vertical ellipsis icon ⋮ to the right of the disk to detach, and then click Detach. When a confirmation message appears, click Detach to confirm.

The data is preserved after the disk is detached from a VM. To delete the PVC that contains the disk data, navigate to Storage → PersistentVolumeClaims, and select the project where the VM is located. Locate the PVC with the VM name followed by the disk name, and then click the vertical ellipsis icon ⋮ at the end of the row. Finally click Delete PersistentVolumeClaim.

A confirmation message indicates that the associated data volume is deleted with the PVC. Click Delete to proceed.

You can also delete the data volume resource by using the following command. The underlying persistent volume claim and persistent volume are removed when you delete the data volume.

[user@host ~]$ oc delete datavolume/mariadb-server-my-disk
datavolume.cdi.kubevirt.io "mariadb-server-my-disk" deleted

After you detach a disk from a VM, you can attach it to a different VM. You can also reattach the disk to the same VM, if needed.

To attach the disk, stop the VM, and navigate to Configuration → Disks. Click Add disk and then complete the form:

Attach an existing PVC to a VM

When OpenShift Virtualization copies a disk image to a PV, it allocates only the space that the image uses.

For example, when you create a 10 GiB blank disk, OpenShift Virtualization does not reserve that space with the back-end storage. Instead, OpenShift Virtualization progressively allocates the space when you start writing data to the disk. This mechanism improves usage of your back-end storage, because it consumes only the space that you use.

However, write speed is often reduced, because the system must allocate the disk blocks before allowing the write operations to proceed.

To preallocate the whole disk space, select the Enable preallocation checkbox when adding a disk to a VM. In a data volume resource file, use the preallocation parameter:

apiVersion: cdi.kubevirt.io/v1beta1
kind: DataVolume
metadata:
  name: datadisk
spec:
  preallocation: true
  storage:
    resources:
      requests:
        storage: 10Gi
    storageClassName: ocs-external-storagecluster-ceph-rbd-​virtualization
  source:
    blank: {}

With this option enabled, OpenShift Virtualization reserves the entire requested space with the back-end storage. This operation consists of writing zeros to the device's blocks for some back-end storage.

Each storage class has a field that specifies whether volume expansion is supported. When volume expansion is available, the storage class resource allowVolumeExpansion field is set to true.

[user@host ~]$ oc get storageclass \
  ocs-external-storagecluster-ceph-rbd-​virtualization -o yaml | \
  grep allowVolumeExpansion
allowVolumeExpansion: true

The storage class does not support volume expansion if the allowVolumeExpansion field is absent or is not set to true.

[user@host ~]$ oc get storageclass nfs-storage -o yaml | \
  grep allowVolumeExpansion	

You can navigate to Storage → StorageClasses, select the name of the storage class, and click YAML to inspect the YAML definition and verify whether the storage class supports volume expansion.

To expand a PVC by using the OpenShift web console, navigate to Storage → PersistentVolumeClaim, select the PVC to edit, click Actions → Expand PVC, type the new size, and click Expand to confirm.

Figure 5.5: Expand a PVC

You can expand a PVC by editing its resource file and modifying the spec.resources.requests.storage parameter.

[user@host ~]$ oc edit pvc/mariadb-server-my-disk-rbd
apiVersion: v1
kind: PersistentVolumeClaim
metadata:
  name: mariadb-server-my-disk-rbd
  namespace: storage-multiple
  ...output omitted...
spec:
  accessModes:
  - ReadWriteMany
  ...output omitted...
  resources:
    requests:
      storage: 8Gi
  storageClassName: ocs-external-storagecluster-ceph-rbd-​virtualization
  volumeMode: Block
  volumeName: pvc-c87ee674-4ddc-3efe-a74e-dfe25da5d7b3
...output omitted...

Resizing a PVC does not require stopping the VM that is using it. However, the VM detects the new disk size only after a restart.

Most operating systems report the new size but do not automatically expand the partitions and the file systems. On Linux systems that enable the cloud-init service, these operations might automatically happen during boot.