Preparing CDI scratch space - Virtual machines | Container-native virtualization

About DataVolumes
Understanding scratch space
CDI operations that require scratch space
Defining a StorageClass in the CDI configuration
CDI supported operations matrix

About DataVolumes

DataVolume objects are custom resources that are provided by the Containerized Data Importer (CDI) project. DataVolumes orchestrate import, clone, and upload operations that are associated with an underlying PersistentVolumeClaim (PVC). DataVolumes are integrated with KubeVirt, 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, the CDI provisions a scratch space PVC equal to the size of the PVC backing the destination DataVolume (DV). The scratch space PVC is deleted after the operation completes or aborts.

The CDIConfig object allows you to define which StorageClass to use to bind the scratch space PVC by setting the scratchSpaceStorageClass in the spec: section of the CDIConfig object.

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

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

Manual provisioning

If there are no storage classes, the CDI will use 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 will remain 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	The 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, the CDI downloads the image to scratch space before passing the file to QEMU-IMG.

Type

Reason

Registry imports

The 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, the CDI downloads the image to scratch space before passing the file to QEMU-IMG.

Defining a StorageClass in the CDI configuration

Define a StorageClass in the CDI configuration to dynamically provision scratch space for CDI operations.

Procedure

Use the oc client to edit the cdiconfig/config and add or edit the spec: scratchSpaceStorageClass to match a StorageClass in the cluster.

$ oc edit cdiconfig/config

API Version:  cdi.kubevirt.io/v1alpha1
kind: CDIConfig
metadata:
  name: config
...
spec:
  scratchSpaceStorageClass: "<storage_class>"
...

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*
Archive+	✓ TAR	✓ TAR	✓ TAR	□ TAR	□ TAR

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*

Archive+

✓ TAR

□ TAR

✓ Supported operation

□ Unsupported operation

* Requires scratch space

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

+ Archive does not support block mode DVs

Additional resources

See the Dynamic provisioning section for more information on StorageClasses and how these are defined in the cluster.