public double getFreeDiskSpace(int dataNodeId) {
    return loadCache.getFreeDiskSpace(dataNodeId);
  }
Get the free disk space of the specified DataNode.

@param dataNodeId The index of the specified DataNode
@return The free disk space that sample through heartbeat, 0 if no heartbeat received
iotdb-core/confignode/src/main/java/org/apache/iotdb/confignode/manager/load/LoadManager.java