Live Migration Switch Over

Live Migration of a virtual machine (VM) from a source server to a target server must happen quickly to avoid interrupting users. This illustration shows how a VM running in production on one server (the source) is transferred to another (the target) when a Live Migration is initiated by an administrator or a management application such as System Center Virtual Machine Manager.

The first step of the Live Migration, labeled “Migration Setup,” transfers configuration information about the VM and associated devices that are running on the source server to the target.

During the second step of the Live Migration, labeled “Memory Transfer,” Hyper-V copies memory pages from the source to the target. This is the most time-consuming step, because the VM on the source is still running, so some pages will change, or become “dirty pages,” after they have been initially copied. Hyper-V repeats the copying process, moving dirty pages from the source to the target until all the pages have been copied, or until it has made 10 passes through the memory pages attempting to complete the copy. If the copy cannot be completed in 10 iterations, Hyper-V suspends the VM on the source and begins the third step. The time the VM is suspended has to be less than the TCP reconnect window, during which time other computers will wait for acknowledgments from the target server without resetting their network connections.