code: public Map<String, Long> getDataSourceSizes()
    {
      return dataSources.entrySet().stream()
                        .collect(Collectors.toMap(Entry::getKey, entry -> entry.getValue().getTotalSegmentSize()));
    }
docs: Returns a map of dataSource to the total byte size of segments managed by this segmentManager.  This method should
  be used carefully because the returned map might be different from the actual data source states.
  
  @return a map of dataSources and their total byte sizes