@Override
    public void run() {
      try {
        serialize();
      } finally {
        if (!isClosed) {
          serializeThread.submit(new SerializeTask());
        }
      }
    }
This task serializes WALEntry to workingBuffer and will call fsync at last.
iotdb-core/datanode/src/main/java/org/apache/iotdb/db/storageengine/dataregion/wal/buffer/WALBuffer.java