Microsoft had added the Hyper-V hypervisor to Windows Server before Release 2, but the R2 version will include Hyper-V virtual machine live migration, a popular feature of virtualized environments that Windows Server was lacking. Live migration, said Laing, can be used to load-balance virtual machines across servers, save power by shutting down a server after moving its virtual machines to an underutilized neighbor, and fail-over the workload of one server to another in the event of a server glitch. "Live migration is the most significant, most visible new feature" of Release 2. Live migration is also a key aspect of implementing better power management for Windows servers. If a server's virtual machines can be shifted from one physical server to another at the click of button, then underutilized servers can offload their VMs and get shut down. The live migration feature can be implemented across up to 16 hosts (physical servers running multiple virtual machines) that share the same storage file system, noted Bill Laing.