@Override
    public Collection<AmbryDisk> getDisks(AmbryDataNode dataNode) {
      if (dataNode != null) {
        final Set<AmbryDisk> disks = ambryDataNodeToAmbryDisks.get(dataNode);
        return disks != null ? disks : Collections.emptyList();
      }
      List<AmbryDisk> disksToReturn = new ArrayList<>();
      for (Set<AmbryDisk> disks : ambryDataNodeToAmbryDisks.values()) {
        disksToReturn.addAll(disks);
      }
      return disksToReturn;
    }
@return the count of datanodes in this cluster that are down.
ambry-clustermap/src/main/java/com/github/ambry/clustermap/HelixClusterManager.java