@Override
  public int getNodesNum() {
    return identifier2Nodes.size();
  }
This strategy creates one wal node for each unique identifier. In other words, each identifier
(like data region) has its own wal node.
iotdb-core/datanode/src/main/java/org/apache/iotdb/db/storageengine/dataregion/wal/allocation/FirstCreateStrategy.java