Take a golden generalized VHDX, with the intention of creating a few billion copies of it. Can this be done in a few milliseconds with deduplication, using no additional space until the VM's are launched? I was googling for this feature and it is described as a btrfs feature in an Arstechnica article, and this is the command in btrfs:

me@server:~$ cp --reflink=always 200GB_virtual_machine_drive.qcow2 clone_of_200GB_virtual_machine_drive.qcow2