@Override
  public int getNodesNum() {
    return walNodes.size();
  }
This strategy creates n wal nodes and allocate them by round-robin strategy. In other words,
several identifiers (like data regions) can share one wal node.
iotdb-core/datanode/src/main/java/org/apache/iotdb/db/storageengine/dataregion/wal/allocation/RoundRobinStrategy.java