code: public void markCompactedSSTablesReplaced(Collection<SSTableReader> oldSSTables, Collection<SSTableReader> allReplacements, OperationType compactionType)
      {
          removeSSTablesFromTracker(oldSSTables);
          releaseReferences(oldSSTables, false);
          notifySSTablesChanged(oldSSTables, allReplacements, compactionType);
          addNewSSTablesSize(allReplacements);
      }
docs: that they have been replaced by the provided sstables, which must have been performed by an earlier replaceReaders() call