long getDownDatanodesCount() {
      long count = 0;
      for (AmbryDataNode datanode : getDatanodes()) {
        if (datanode.getState() == HardwareState.UNAVAILABLE) {
          count++;
        }
      }
      return count;
    }
@return the count of datanodes in this cluster that are down.
ambry-clustermap/src/main/java/com/github/ambry/clustermap/HelixClusterManager.java