A mapping layer for filesystems

Err, really? Thin provisioning of storage for VMs is incredibly powerful and flexible and not a kludge. Not everything is ext4 or XFS, and thin provisioning allows VM admins to not interact with the VMs themselves (for example, the VMs can stay up!). Also, I don’t necessarily have root on those VMs, and I sure as hell don’t want to have it.

Are you talking about some more specific instance of thin provisioning?