Like the PC and Internet revolutions before, virtualization software has changed the world. End-users can carry around several operating system (OS) versions and platforms on a single computer. Single applications can be virtualized all the way up to entire desktops. Data centers, which used to be composed of hundreds of single-purpose computers, are now represented by much smaller, and more inexpensive, clusters of virtualization hosts. Now it is common to have dozens to hundreds of guest images running on a single host. Or to have multiple, incompatible, virtual applications running side-by-side, with the end-user unaware of all the supporting technology needed to pull it off.