code: public void unreferenceSSTables()
      {
          Set<SSTableReader> notCompacting;
  
          View currentView, newView;
          do
          {
              currentView = view.get();
              notCompacting = currentView.nonCompactingSStables();
              newView = currentView.replace(notCompacting, Collections.<SSTableReader>emptySet());
          }
          while (!view.compareAndSet(currentView, newView));
  
          if (notCompacting.isEmpty())
          {
              // notifySSTablesChanged -> LeveledManifest.promote doesn't like a no-op "promotion"
              return;
          }
          notifySSTablesChanged(notCompacting, Collections.<SSTableReader>emptySet(), OperationType.UNKNOWN);
          removeOldSSTablesSize(notCompacting);
          releaseReferences(notCompacting, true);
      }
docs: removes all sstables that are not busy compacting.