Data Movement Policies:
The data movement policies are based on when a file was last modified. You can set policies with dates and times that differ for subsets of data. Files that may be updated for a period of time need a different policy than ones that never change. Data movement policies are set on a per-MTree basis.
The data is moved from the active tier to the retention tier as set by your data movement policies. Data in the retention tier can be deleted. Prior to DD OS 5.2, you could not reclaim this freed-up space until all data on a retention unit was completely deleted, and then you had to reclaim the entire unit.
Starting with DD OS 5.3, you can reclaim this freed-up space.
Space reclamation runs in the background as a low-priority activity. It suspends itself when there are higher priority activities, such as data movement and cleaning. It reclaims space from deleted data across all units in the retention tier, migrates data between retention units to keep the older retention units full, and moves the free space to the target retention unit.