Persistent data of virtual machines are migrated between and across data stores using a method that provides for a bounded disk size, low performance overhead, arbitrarily low downtimes, atomic switch-over, and ability to build optimizations based around block level heuristics. The method includes the steps of copying the persistent data at the source data store to the destination data store, updating a bitmap data structure during the copying step to indicate which blocks of the persistent data have been modified during the copying step, identifying the blocks that have been modified during the copying step using the bitmap data structure, and copying the identified blocks to the destination data store. The method may further include the step of determining that the number of the identified blocks is below a predetermined threshold and suspending the virtual machine prior to the second copying step.