Using container disks with virtual machines

About container disks
- Importing a container disk into a PVC by using a data volume
- Attaching a container disk to a virtual machine as a containerDisk volume
Preparing a container disk for virtual machines
Disabling TLS for a container registry to use as insecure registry
Next steps

You can build a virtual machine image into a container disk and store it in your container registry. You can then import the container disk into persistent storage for a virtual machine or attach it directly to the virtual machine for ephemeral storage.

If you use large container disks, I/O traffic might increase, impacting worker nodes. This can lead to unavailable nodes. You can resolve this by:

About container disks

A container disk is a virtual machine image that is stored as a container image in a container image registry. You can use container disks to deliver the same disk images to multiple virtual machines and to create large numbers of virtual machine clones.

A container disk can either be imported into a persistent volume claim (PVC) by using a data volume that is attached to a virtual machine, or attached directly to a virtual machine as an ephemeral containerDisk volume.

Importing a container disk into a PVC by using a data volume

Use the Containerized Data Importer (CDI) to import the container disk into a PVC by using a data volume. You can then attach the data volume to a virtual machine for persistent storage.

Attaching a container disk to a virtual machine as a `containerDisk` volume

A containerDisk volume is ephemeral. It is discarded when the virtual machine is stopped, restarted, or deleted. When a virtual machine with a containerDisk volume starts, the container image is pulled from the registry and hosted on the node that is hosting the virtual machine.

Use containerDisk volumes for read-only file systems such as CD-ROMs or for disposable virtual machines.

Using containerDisk volumes for read-write file systems is not recommended because the data is temporarily written to local storage on the hosting node. This slows live migration of the virtual machine, such as in the case of node maintenance, because the data must be migrated to the destination node. Additionally, all data is lost if the node loses power or otherwise shuts down unexpectedly.

Preparing a container disk for virtual machines

You must build a container disk with a virtual machine image and push it to a container registry before it can used with a virtual machine. You can then either import the container disk into a PVC using a data volume and attach it to a virtual machine, or you can attach the container disk directly to a virtual machine as an ephemeral containerDisk volume.

The size of a disk image inside a container disk is limited by the maximum layer size of the registry where the container disk is hosted.

For Red Hat Quay, you can change the maximum layer size by editing the YAML configuration file that is created when Red Hat Quay is first deployed.

Prerequisites

Install podman if it is not already installed.
The virtual machine image must be either QCOW2 or RAW format.

Procedure

Create a Dockerfile to build the virtual machine image into a container image. The virtual machine image must be owned by QEMU, which has a UID of 107, and placed in the /disk/ directory inside the container. Permissions for the /disk/ directory must then be set to 0440.

The following example uses the Red Hat Universal Base Image (UBI) to handle these configuration changes in the first stage, and uses the minimal scratch image in the second stage to store the result:
```
$ cat > Dockerfile << EOF
FROM registry.access.redhat.com/ubi8/ubi:latest AS builder
ADD --chown=107:107 <vm_image>.qcow2 /disk/ (1)
RUN chmod 0440 /disk/*

FROM scratch
COPY --from=builder /disk/* /disk/
EOF
```
1 Where <vm_image> is the virtual machine image in either QCOW2 or RAW format.
To use a remote virtual machine image, replace <vm_image>.qcow2 with the complete url for the remote image.

Build and tag the container:

$ podman build -t <registry>/<container_disk_name>:latest .

Push the container image to the registry:

$ podman push <registry>/<container_disk_name>:latest

If your container registry does not have TLS you must add it as an insecure registry before you can import container disks into persistent storage.

Disabling TLS for a container registry to use as insecure registry

You can disable TLS (transport layer security) for a container registry by adding the registry to the cdi-insecure-registries config map.