Preparing CDI scratch space - Virtual machines | OpenShift Virtualization

About data volumes
Understanding scratch space
CDI operations that require scratch space
Defining a storage class
CDI supported operations matrix
Additional resources

About data volumes

DataVolume objects are custom resources that are provided by the Containerized Data Importer (CDI) project. Data volumes orchestrate import, clone, and upload operations that are associated with an underlying persistent volume claim (PVC). Data volumes are integrated with OpenShift Virtualization, and they prevent a virtual machine from being started before the PVC has been prepared.

Understanding scratch space

The Containerized Data Importer (CDI) requires scratch space (temporary storage) to complete some operations, such as importing and uploading virtual machine images. During this process, CDI provisions a scratch space PVC equal to the size of the PVC backing the destination data volume (DV). The scratch space PVC is deleted after the operation completes or aborts.

You can define the storage class that is used to bind the scratch space PVC in the spec.scratchSpaceStorageClass field of the HyperConverged custom resource.

If the defined storage class does not match a storage class in the cluster, then the default storage class defined for the cluster is used. If there is no default storage class defined in the cluster, the storage class used to provision the original DV or PVC is used.

CDI requires requesting scratch space with a file volume mode, regardless of the PVC backing the origin data volume. If the origin PVC is backed by block volume mode, you must define a storage class capable of provisioning file volume mode PVCs.

Manual provisioning

If there are no storage classes, CDI uses any PVCs in the project that match the size requirements for the image. If there are no PVCs that match these requirements, the CDI import pod remains in a Pending state until an appropriate PVC is made available or until a timeout function kills the pod.

CDI operations that require scratch space

Type	Reason
Registry imports	CDI must download the image to a scratch space and extract the layers to find the image file. The image file is then passed to QEMU-IMG for conversion to a raw disk.
Upload image	QEMU-IMG does not accept input from STDIN. Instead, the image to upload is saved in scratch space before it can be passed to QEMU-IMG for conversion.
HTTP imports of archived images	QEMU-IMG does not know how to handle the archive formats CDI supports. Instead, the image is unarchived and saved into scratch space before it is passed to QEMU-IMG.
HTTP imports of authenticated images	QEMU-IMG inadequately handles authentication. Instead, the image is saved to scratch space and authenticated before it is passed to QEMU-IMG.
HTTP imports of custom certificates	QEMU-IMG inadequately handles custom certificates of HTTPS endpoints. Instead, CDI downloads the image to scratch space before passing the file to QEMU-IMG.

Type

Reason

Registry imports

CDI must download the image to a scratch space and extract the layers to find the image file. The image file is then passed to QEMU-IMG for conversion to a raw disk.

Upload image

QEMU-IMG does not accept input from STDIN. Instead, the image to upload is saved in scratch space before it can be passed to QEMU-IMG for conversion.

HTTP imports of archived images

QEMU-IMG does not know how to handle the archive formats CDI supports. Instead, the image is unarchived and saved into scratch space before it is passed to QEMU-IMG.

HTTP imports of authenticated images

QEMU-IMG inadequately handles authentication. Instead, the image is saved to scratch space and authenticated before it is passed to QEMU-IMG.

HTTP imports of custom certificates

QEMU-IMG inadequately handles custom certificates of HTTPS endpoints. Instead, CDI downloads the image to scratch space before passing the file to QEMU-IMG.

Defining a storage class

You can define the storage class that the Containerized Data Importer (CDI) uses when allocating scratch space by adding the spec.scratchSpaceStorageClass field to the HyperConverged custom resource (CR).

Prerequisites

Install the OpenShift CLI (oc).

Procedure

Edit the HyperConverged CR by running the following command:
```
$ oc edit hco -n openshift-cnv kubevirt-hyperconverged
```
Add the spec.scratchSpaceStorageClass field to the CR, setting the value to the name of a storage class that exists in the cluster:
```
apiVersion: hco.kubevirt.io/v1beta1
kind: HyperConverged
metadata:
  name: kubevirt-hyperconverged
spec:
  scratchSpaceStorageClass: "<storage_class>" (1)
```
1 If you do not specify a storage class, CDI uses the storage class of the persistent volume claim that is being populated.
Save and exit your default editor to update the HyperConverged CR.

CDI supported operations matrix

This matrix shows the supported CDI operations for content types against endpoints, and which of these operations requires scratch space.

Content types	HTTP	HTTPS	HTTP basic auth	Registry	Upload
KubeVirt (QCOW2)	✓ QCOW2 ✓ GZ* ✓ XZ*	✓ QCOW2** ✓ GZ* ✓ XZ*	✓ QCOW2 ✓ GZ* ✓ XZ*	✓ QCOW2* □ GZ □ XZ	✓ QCOW2* ✓ GZ* ✓ XZ*
KubeVirt (RAW)	✓ RAW ✓ GZ ✓ XZ	✓ RAW ✓ GZ ✓ XZ	✓ RAW ✓ GZ ✓ XZ	✓ RAW* □ GZ □ XZ	✓ RAW* ✓ GZ* ✓ XZ*

Content types

HTTP

HTTPS

HTTP basic auth

Registry

Upload

KubeVirt (QCOW2)

✓ QCOW2
✓ GZ*
✓ XZ*

✓ QCOW2**
✓ GZ*
✓ XZ*

✓ QCOW2
✓ GZ*
✓ XZ*

✓ QCOW2*
□ GZ
□ XZ

✓ QCOW2*
✓ GZ*
✓ XZ*

KubeVirt (RAW)

✓ RAW
✓ GZ
✓ XZ

✓ RAW*
□ GZ
□ XZ

✓ RAW*
✓ GZ*
✓ XZ*

✓ Supported operation

□ Unsupported operation

* Requires scratch space

** Requires scratch space if a custom certificate authority is required

Additional resources

Dynamic provisioning