Another union filesystem approach

if your underlying filesystem is a default system image and your union is then a specific system, it makes a huge amount of sense to want the ability to update the underlying filesystem and have everything using a union pick up the changes.

you may be able to unmount the overlay, but then when you re-mount it, how do you know what to do to resolve changes? In some cases you may want the new file from the underlying layer, in some cases you want the modified version from the top layer, and in many cases what you really want is the changes that were made between the old underlying file and the old upper layer to be made to the new underlying file into the new upper layer.