private void startTimedService() {
    // timed flush sequence memtable
    if (CONFIG.isEnableTimedFlushSeqMemtable()) {
      seqMemtableTimedFlushCheckThread =
          IoTDBThreadPoolFactory.newSingleThreadScheduledExecutor(
              ThreadName.TIMED_FLUSH_SEQ_MEMTABLE.getName());
      ScheduledExecutorUtil.safelyScheduleAtFixedRate(
          seqMemtableTimedFlushCheckThread,
          this::timedFlushSeqMemTable,
          CONFIG.getSeqMemtableFlushCheckInterval(),
          CONFIG.getSeqMemtableFlushCheckInterval(),
          TimeUnit.MILLISECONDS);
      LOGGER.info("start sequence memtable timed flush check thread successfully.");
    }
    // timed flush unsequence memtable
    if (CONFIG.isEnableTimedFlushUnseqMemtable()) {
      unseqMemtableTimedFlushCheckThread =
          IoTDBThreadPoolFactory.newSingleThreadScheduledExecutor(
              ThreadName.TIMED_FLUSH_UNSEQ_MEMTABLE.getName());
      ScheduledExecutorUtil.safelyScheduleAtFixedRate(
          unseqMemtableTimedFlushCheckThread,
          this::timedFlushUnseqMemTable,
          CONFIG.getUnseqMemtableFlushCheckInterval(),
          CONFIG.getUnseqMemtableFlushCheckInterval(),
          TimeUnit.MILLISECONDS);
      LOGGER.info("start unsequence memtable timed flush check thread successfully.");
    }
  }
get StorageGroup -> DataRegionIdList map from data/system directory.
iotdb-core/datanode/src/main/java/org/apache/iotdb/db/storageengine/StorageEngine.java