Cockpit Machines warns in certain situations when a disk image is alredy used by a VM. This is of course done to avoid destroying data.

However, external snapshots create disk images that are used as qcow2 backing files for other images and are not themselves mentioned directly in the XML of the VM. These images are nevertheless used by the VM and are just as important as the ones that link to them.

Cockpit Machines needs to follow the backing file chains to find all images that are directly and indirectly used by a VM so that it can properly want the user about destroying them.