On Tue, Nov 30, 2021 at 14:51:54 +0100, Elias Mobery wrote:
> Hello Peter, thank you so much for that detailed info!
>
> Sorry, you were right, when trying to delete my external snapshot via
> snapshot-delete, the error says "deletion of external snapshots unsupported"
>
> I can't merge the snapshot because the VM image is in a read-only
> filesystem. Sorry I should've said, it's a live system. So the image is in
> the read-only squashfs and external snapshot in overlay is used for writing.

Okay, that changes the situation quite a bit:

>
> Now I would like the snapshot emptied or deleted/recreated when it reaches
> 4GB.
>
> Is there even a way to do this with the image being read-only?

If the base image is on a read-only filesystem you obviously can't
commit to it.

Now the question is what should happen to the data in the overlay.

Discarding/recreating the overlay image is possible only once you turn
off the VM because it basically rolls back the state of the disk back to
the time when the overlay was created. This means that everything
written to the disk will be lost.

Filesystems obviously can't handle that so that's why it simply won't be
possible to do live with the root image.

You can have a second disk, which you hot-unplug, wipe the overlay and
plug it back.

Another possibility is to enable trim/discard and just simply delete the
data in the VM which was added after the overlay was created. When
trim/discard is enabled on all layers incluging the guest filesystem,
then deleting stuff inside the VM will also mean that the overlay will
shrink again.