Creating VMs by cloning PVCs

You can create virtual machines (VMs) by cloning existing persistent volume claims (PVCs) with custom images.

You must install the QEMU guest agent on VMs created from operating system images that are not provided by Red Hat.

You clone a PVC by creating a data volume that references a source PVC.

About cloning

When cloning a data volume, the Containerized Data Importer (CDI) chooses one of the Container Storage Interface (CSI) clone methods: CSI volume cloning or smart cloning. Both methods are efficient but have certain requirements. If the requirements are not met, the CDI uses host-assisted cloning.

Host-assisted cloning is the slowest and least efficient method of cloning, but it has fewer requirements than either of the other two cloning methods.

CSI volume cloning

Container Storage Interface (CSI) cloning uses CSI driver features to more efficiently clone a source data volume.

CSI volume cloning has the following requirements:

The CSI driver that backs the storage class of the persistent volume claim (PVC) must support volume cloning.
For provisioners not recognized by the CDI, the corresponding storage profile must have the cloneStrategy set to CSI Volume Cloning.
The source and target PVCs must have the same storage class and volume mode.
If you create the data volume, you must have permission to create the datavolumes/source resource in the source namespace.
The source volume must not be in use.

Smart cloning

When a Container Storage Interface (CSI) plugin with snapshot capabilities is available, the Containerized Data Importer (CDI) creates a persistent volume claim (PVC) from a snapshot, which then allows efficient cloning of additional PVCs.

Smart cloning has the following requirements:

A snapshot class associated with the storage class must exist.
The source and target PVCs must have the same storage class and volume mode.
If you create the data volume, you must have permission to create the datavolumes/source resource in the source namespace.
The source volume must not be in use.

Host-assisted cloning

When the requirements for neither Container Storage Interface (CSI) volume cloning nor smart cloning have been met, host-assisted cloning is used as a fallback method. Host-assisted cloning is less efficient than either of the two other cloning methods.

Host-assisted cloning uses a source pod and a target pod to copy data from the source volume to the target volume. The target persistent volume claim (PVC) is annotated with the fallback reason that explains why host-assisted cloning has been used, and an event is created.

Example PVC target annotation:

apiVersion: v1
kind: PersistentVolumeClaim
metadata:
  annotations:
    cdi.kubevirt.io/cloneFallbackReason: The volume modes of source and target are incompatible
    cdi.kubevirt.io/clonePhase: Succeeded
    cdi.kubevirt.io/cloneType: copy

Example event:

NAMESPACE   LAST SEEN   TYPE      REASON                    OBJECT                              MESSAGE
test-ns     0s          Warning   IncompatibleVolumeModes   persistentvolumeclaim/test-target   The volume modes of source and target are incompatible

Creating a VM from a PVC by using the web console

You can create a virtual machine (VM) by cloning a persistent volume claim (PVC) by using the OpenShift Container Platform web console.

Prerequisites

You must have access to the namespace that contains the source PVC.

Procedure

Navigate to Virtualization → Catalog in the web console.
Click a template tile without an available boot source.
Click Customize VirtualMachine.
On the Customize template parameters page, expand Storage and select PVC (clone PVC) from the Disk source list.
Select the PVC project and the PVC name.
Set the disk size.
Click Next.
Click Create VirtualMachine.

Creating a VM from a PVC by using the CLI

You can create a virtual machine (VM) by cloning the persistent volume claim (PVC) of an existing VM by using the command line.

You can clone a PVC by using one of the following options:

Cloning a PVC to a new data volume.

This method creates a data volume whose lifecycle is independent of the original VM. Deleting the original VM does not affect the new data volume or its associated PVC.
Cloning a PVC by creating a VirtualMachine manifest with a dataVolumeTemplates stanza.

This method creates a data volume whose lifecycle is dependent on the original VM. Deleting the original VM deletes the cloned data volume and its associated PVC.

Optimizing clone Performance at scale in OpenShift Data Foundation

When you use OpenShift Data Foundation, the storage profile configures the default cloning strategy as csi-clone. However, this method has limitations, as shown in the following link.

After a certain number of clones are created from a persistent volume claim (PVC), a background flattening process begins, which can significantly reduce clone creation performance at scale.

To improve performance when creating hundreds of clones from a single source PVC, use the VolumeSnapshot cloning method instead of the default csi-clone strategy.

Procedure

Create a VolumeSnapshot custom resource (CR) of the source image by using the following content:

apiVersion: snapshot.storage.k8s.io/v1
kind: VolumeSnapshot
metadata:
  name: golden-volumesnapshot
  namespace: golden-ns
spec:
  volumeSnapshotClassName: ocs-storagecluster-rbdplugin-snapclass
  source:
    persistentVolumeClaimName: golden-snap-source

Add the spec.source.snapshot stanza to reference the VolumeSnapshot as the source for the DataVolume clone:
```
spec:
  source:
    snapshot:
      namespace: golden-ns
      name: golden-volumesnapshot
```

Additional resources

Cloning a PVC to a data volume

You can clone the persistent volume claim (PVC) of an existing virtual machine (VM) disk to a data volume by using the command line.

You create a data volume that references the original source PVC. The lifecycle of the new data volume is independent of the original VM. Deleting the original VM does not affect the new data volume or its associated PVC.

Cloning between different volume modes is supported for host-assisted cloning, such as cloning from a block persistent volume (PV) to a file system PV, as long as the source and target PVs belong to the kubevirt content type.

Note

Smart-cloning is faster and more efficient than host-assisted cloning because it uses snapshots to clone PVCs. Smart-cloning is supported by storage providers that support snapshots, such as Red Hat OpenShift Data Foundation.

Cloning between different volume modes is not supported for smart-cloning.

Prerequisites

You have installed the OpenShift CLI (oc).
The VM with the source PVC must be powered down.
If you clone a PVC to a different namespace, you must have permissions to create resources in the target namespace.
Additional prerequisites for smart-cloning:
- Your storage provider must support snapshots.
- The source and target PVCs must have the same storage provider and volume mode.
- The value of the driver key of the VolumeSnapshotClass object must match the value of the provisioner key of the StorageClass object as shown in the following example:
  
  Example VolumeSnapshotClass object:
  kind: VolumeSnapshotClass apiVersion: snapshot.storage.k8s.io/v1 driver: openshift-storage.rbd.csi.ceph.com # ...
  Example StorageClass object:
  kind: StorageClass apiVersion: storage.k8s.io/v1 # ... provisioner: openshift-storage.rbd.csi.ceph.com

Procedure

Create a DataVolume manifest as shown in the following example:

apiVersion: cdi.kubevirt.io/v1beta1
kind: DataVolume
metadata:
  name: <datavolume> 
spec:
  source:
    pvc:
      namespace: "<source_namespace>" 
      name: "<my_vm_disk>" 
  storage: {}

Specify the name of the new data volume.
Specify the namespace of the source PVC.
Specify the name of the source PVC.

Create the data volume by running the following command:
```
$ oc create -f <datavolume>.yaml
```
Note

Data volumes prevent a VM from starting before the PVC is prepared. You can create a VM that references the new data volume while the PVC is being cloned.

Creating a VM from a cloned PVC by using a data volume template

You can create a virtual machine (VM) that clones the persistent volume claim (PVC) of an existing VM by using a data volume template. This method creates a data volume whose lifecycle is independent on the original VM.

Prerequisites

The VM with the source PVC must be powered down.
You have installed the virtctl CLI.
You have installed the OpenShift CLI (oc).

Procedure

Create a VirtualMachine manifest for your VM and save it as a YAML file, for example:

$ virtctl create vm --name rhel-9-clone --volume-import type:pvc,src:my-project/imported-volume-q5pr9

Review the VirtualMachine manifest for your VM:

apiVersion: kubevirt.io/v1
kind: VirtualMachine
metadata:
  name: rhel-9-clone 
spec:
  dataVolumeTemplates:
  - metadata:
      name: imported-volume-h4qn8
    spec:
      source:
        pvc:
          name: imported-volume-q5pr9 
          namespace: my-project 
      storage:
        resources: {}
  instancetype:
    inferFromVolume: imported-volume-h4qn8 
    inferFromVolumeFailurePolicy: Ignore
  preference:
    inferFromVolume: imported-volume-h4qn8 
    inferFromVolumeFailurePolicy: Ignore
  runStrategy: Always
  template:
    spec:
      domain:
        devices: {}
        memory:
          guest: 512Mi
        resources: {}
      terminationGracePeriodSeconds: 180
      volumes:
      - dataVolume:
          name: imported-volume-h4qn8
        name: imported-volume-h4qn8

The VM name.
The name of the source PVC.
The namespace of the source PVC.
If the PVC source has appropriate labels, the instance type is inferred from the selected DataSource object.
If the PVC source has appropriate labels, the preference is inferred from the selected DataSource object.

Create the virtual machine with the PVC-cloned data volume:
```
$ oc create -f <vm_manifest_file>.yaml
```