Druid 0.9.0 - Druid coordinator automatic segment merging. How does the data flow?

I know this much

  1. Coordinator tells which segments need to be merged
  2. Overlord reads in all these merge requests and distributes it to middle managers
  3. Middle Managers use peon to do individual requests
  4. After merge the segment is handed over to historical nodes
    After that what happens? When does historical node send it to deep storage?