Virtual machines, that is, programs that emulate physical computers, can be stored, along with guest operating systems and programs running on the guest operating system, as virtual-machine images.  Virtual machine images are files and, as such, can be stored, replicated, and transferred.  Thus, when a virtual machine has been successfully created or updated, instances of it can be distributed, e.g., to other physical hosts.  For example, a new or updated virtual machine can be replicated, and the copies can be distributed to geographically separate data centers to provide better access for geographically distributed users.