The tool is similar to Balancer. It periodic scans the blocks in HDFS and uses path and/or other meta data (e.g. mtime) to determine if a block should be cooled down (i.e. hot => warm, or warm => cold) or warmed up (i.e. cold => warm, or warm => hot). In contrast to Balancer, the migration tool always move replicas to a different storage type. Similar to Balancer, the replicas are moved in a way that the number of racks the block does not decrease.