long getDiskDownCount() {
    long diskDownCount = 0;
    for (MockHelixAdmin helixAdmin : helixAdmins.values()) {
      for (String instanceName : helixAdmin.getDownInstances()) {
        diskDownCount += helixAdmin.getDiskCountOnNode(instanceName);
      }
    }
    return diskDownCount;
  }
@return Count of the number of disks in total in this cluster that are down.
ambry-clustermap/src/test/java/com/github/ambry/clustermap/MockHelixCluster.java