In a hierarchical storage system, blocks of data selected for auto-tiering migration, are selected based on dynamically adjusted group sizes. Contiguous blocks are organized into default groups. I/O activity of the blocks in a group is monitored. Based on the I/O activity, the default groups may be sub-divided into smaller sub-groups or combined into larger groups, to separate as much as practical, contiguous series of cooler blocks and contiguous series of hotter blocks into respective focused (concentrated) groups or sub-groups. The concentrated group or sub-group may then be migrated according to the average I/O activity of the included blocks. Group configurations are continually and dynamically adjusted according to changing I/O conditions.