Evaggelos Balaskas - System Engineer

When creating a new Cloud Virtual Machine the cloud provider is copying a virtual disk as the base image (we called it mí̱tra or matrix) and starts your virtual machine from another virtual disk (or volume cloud disk) that in fact is a snapshot of the base image.

Just for the sake of this example, let us say that the base cloud image is the

jammy-server-cloudimg-amd64.img

When creating a new Libvirt (qemu/kvm) virtual machine, you can use this base image to start your VM instead of using an iso to install ubuntu 22.04 LTS. When choosing this image, then all changes will occur to that image and if you want to spawn another virtual machine, you need to (re)download it.

So instead of doing that, the best practice is to copy this image as base and start from a snapshot aka a baking file from that image. It is best because you can always quickly revert all your changes and (re)spawn the VM from the fresh/clean base image. Or you can always create another snapshot and revert if needed.

inspect images

To see how that works here is a local example from my linux machine.

qemu-img info /var/lib/libvirt/images/lEvXLA_tf-base.qcow2

image: /var/lib/libvirt/images/lEvXLA_tf-base.qcow2
file format: qcow2
virtual size: 2.2 GiB (2361393152 bytes)
disk size: 636 MiB
cluster_size: 65536
Format specific information:
    compat: 0.10
    compression type: zlib
    refcount bits: 16

the most important attributes to inspect are

virtual size: 2.2 GiB
disk size: 636 MiB

and the volume disk of my virtual machine

qemu-img info /var/lib/libvirt/images/lEvXLA_tf-vol.qcow2

image: /var/lib/libvirt/images/lEvXLA_tf-vol.qcow2
file format: qcow2
virtual size: 10 GiB (10737418240 bytes)
disk size: 1.6 GiB
cluster_size: 65536
backing file: /var/lib/libvirt/images/lEvXLA_tf-base.qcow2
backing file format: qcow2
Format specific information:
    compat: 0.10
    compression type: zlib
    refcount bits: 16

We see here

virtual size: 10 GiB
disk size: 1.6 GiB

cause I have extended the volume disk size to 10G from 2.2G , doing some updates and install some packages.

Now here is a problem.

I would like to use my own cloud image as base for some projects. It will help me speed things up and also do some common things I am usually doing in every setup.

If I copy the volume disk, then I will copy 1.6G of the snapshot disk. I can not use this as a base image. The volume disk contains only the delta from the base image!

baking file

Let’s first understand a bit better what is happening here

qemu-img info –backing-chain /var/lib/libvirt/images/lEvXLA_tf-vol.qcow2

image: /var/lib/libvirt/images/lEvXLA_tf-vol.qcow2
file format: qcow2
virtual size: 10 GiB (10737418240 bytes)
disk size: 1.6 GiB
cluster_size: 65536
backing file: /var/lib/libvirt/images/lEvXLA_tf-base.qcow2
backing file format: qcow2
Format specific information:
    compat: 0.10
    compression type: zlib
    refcount bits: 16

image: /var/lib/libvirt/images/lEvXLA_tf-base.qcow2
file format: qcow2
virtual size: 2.2 GiB (2361393152 bytes)
disk size: 636 MiB
cluster_size: 65536
Format specific information:
    compat: 0.10
    compression type: zlib
    refcount bits: 16

By inspecting the volume disk, we see that this image is chained to our base image.

disk size: 1.6 GiB
disk size: 636 MiB

Commit Volume

If we want to commit our volume changes to our base images, we need to commit them.

sudo qemu-img commit /var/lib/libvirt/images/lEvXLA_tf-vol.qcow2

Image committed.

Be aware, we commit our changes the volume disk => so our base will get the updates !!

Base Image

We need to see our base image grow we our changes

  disk size: 1.6 GiB
+ disk size: 636 MiB
=
  disk size: 2.11 GiB

and we can verify that by getting the image info (details)

qemu-img info /var/lib/libvirt/images/lEvXLA_tf-base.qcow2

image: /var/lib/libvirt/images/lEvXLA_tf-base.qcow2
file format: qcow2
virtual size: 10 GiB (10737418240 bytes)
disk size: 2.11 GiB
cluster_size: 65536
Format specific information:
    compat: 0.10
    compression type: zlib
    refcount bits: 16

That’s it !