It's too easy to use a stale/obsolete rootfs tarball
If I run vmdb2 and it creates a tarball, and I then change one of the steps that would result in a different tarball (e.g., I change the Debian release used for debootstrap), vmdb2 uses the same tarball even without a warning.
It'd be handy if vmdb2 could name the tarball automatically, based on a hash of the steps between unpacking and creating a tarball.
This may end up resulting a large numbers of tarballs, of course.