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OpenBSD vmm and qcow2 derived disks

Written by Solène, on 27 August 2023.
Tags: #openbsd

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1. Introduction §

Let me show you a very practical feature of qcow2 virtual disk format, that is available in OpenBSD vmm, allowing you to easily create derived disks from an original image (also called delta disks).

A derived disk image is a new storage file that will inherit all the data from the original file, without modifying the original ever, it's like stacking a new fresh disk on top of the previous one, but all the changes are now written on the new one.

This allows interesting use cases such as using a golden image to provide a base template, like a fresh OpenBSD install, or create a temporary disks to try changes without harming to original file (and without having to backup a potentially huge file).

This is NOT OpenBSD specific, it's a feature of the qcow2 format, so while this guide is using OpenBSD as an example, this will work wherever qcow2 can be used.

OpenBSD vmctl man page: -b flag

2. Setup §

First, you need to have a qcow2 file with something installed in it, let's say you already have a virtual machine with its storage file /var/lib/vmm/alpine.qcow2.

We will create a derived file /var/lib/vmm/derived.qcow2 using the vmctl command:

# vmctl create -b /var/lib/vmm/alpine.qcow2 /var/lib/vmm/derived.qcow2

That's it! Now you have the new disk that already inherits all the other file data without modifying it ever.

3. Limitations §

The derived disk will stop working if the original file is modified, so once you make derived disks from a base image, you shouldn't modify the base image.

However, it's possible to merge changes from a derived disk to the base image using the qemu-img command:

Red Hat documentation: Rebasing a Backing File of an Image

4. Conclusion §

The derived images can be useful in some scenarios, if you have an image and want to make some experimentation without making a full backup, just use a derived disk. If you want to provide a golden image as a start like an installed OS, this will work too.

One use case I had was with OpenKuBSD, I had a single OpenBSD install as a base image, each VM had a derived disk as their root but removed and recreated at every boot, but they also had a dedicated disk for /home, this allows me to keep all the VMs clean, and I just have a single system to manage.