Remember:

This garbage-collection process then requires a certain amount of free space to work efficiently. For this reason it is recommended to keep approximately 15% free space in a DRP pool.

Note: Where possible limit the maximum transfer size sent to the IBM FlashSystem to no more than 256 KiB. This limitation is general best practice and not specific to only DRP. Based on perf issue, IBM recommend to use 64kB IO, for volumes with dedup^and compression.