@Override
  public void shutdown(long milliseconds) throws ShutdownException {
    try {
      for (DataRegion dataRegion : dataRegionMap.values()) {
        if (dataRegion != null) {
          CompactionScheduleTaskManager.getInstance().unregisterDataRegion(dataRegion);
        }
      }
      forceCloseAllProcessor();
    } catch (TsFileProcessorException e) {
      throw new ShutdownException(e);
    }
    shutdownTimedService(seqMemtableTimedFlushCheckThread, "SeqMemtableTimedFlushCheckThread");
    shutdownTimedService(unseqMemtableTimedFlushCheckThread, "UnseqMemtableTimedFlushCheckThread");
    cachedThreadPool.shutdownNow();
    dataRegionMap.clear();
  }
get StorageGroup -> DataRegionIdList map from data/system directory.
iotdb-core/datanode/src/main/java/org/apache/iotdb/db/storageengine/StorageEngine.java