long getDiskCount() {
      long count = 0;
      for (Set<AmbryDisk> disks : ambryDataNodeToAmbryDisks.values()) {
        count += disks.size();
      }
      return count;
    }
@return the count of disks in this cluster.
ambry-clustermap/src/main/java/com/github/ambry/clustermap/HelixClusterManager.java