Virtual Machines (VMs) refer to the software implementation of a computer that runs its own OS and applications as if it was a Physical Machine (PM). Live migration of VMs allows a server administrator to move a running virtual machine or application among different physical machines without disconnecting the client or application. Total migration time and downtime are two key performance metrics that the clients of a VM service care about the most, because they are concerned about service degradation and the duration that the service is completely unavailable.