Techniques for migrating persistent data of virtual machines between and across data stores are optimized using special tracking data structures and monitoring methods. Special tracking data structures include an incremental change block tracking bitmap that indicate what blocks have been modified during a copy operation. The determination of whether any one block has been modified during the copy operation is based on whether or not the copy operation has progressed past that block. Another special tracking data structure is a Bloom filter, which provides a space-efficient data structure for keeping track of dirtied blocks. In addition, heat-based optimization techniques are applied so that blocks that are frequently updated are filtered and not transferred to the destination data store until the last iteration of the migration process.