Instead of working on file-level devices, this driver operates directly on the block device where your Docker instance is. While the default setup generally sets up a loopback device and is mostly fine for local testing, this particular setup is extremely not suggested for production systems due to the sparse files it creates in the loopback device. For production systems, you are encouraged to combine it with direct-lvm, but that kind of intricate setup requires a configuration that is particularly tricky and slower than the overlay storage driver, so I would generally not recommend its use unless you are unable to use aufs or overlay/overlay2.