1. A source system, comprising: memory; and a hardware processor executing code to cause at least some of said memory to be migrated over a network to a target system; wherein the processor causes said memory to be iteratively migrated by migrating some of said memory while a guest continues to write said memory, halting execution of said guest, and completing a remainder of the memory migration; and wherein for each iteration, the processor determines which units of memory are marked as dirty on an inactive bitmap and, for each unit of memory, if the unit of memory is marked as dirty on the inactive bitmap, then the processor migrates that unit of memory if a corresponding bit on an active bitmap does not indicate the unit of memory is dirty and does not migrate the unit of memory if the corresponding bit on the active bit map indicates the unit of memory to be dirty.