1. A method for migrating a virtual machine disk (VM disk) from first physical storage to second physical storage while the virtual machine (VM) is running, the method comprising: taking a snapshot of the VM disk as represented by a first parent VM disk stored on the first physical storage, whereby a first child VM disk is created on physical storage; copying the first parent VM disk to the second physical storage as a second parent VM disk; re-parenting the first VM child disk to the second parent VM disk; and consolidating the first child VM disk and the second parent VM disk.