@Override
  default int compareTo(DataNodeId o) {
    if (o == null) {
      throw new NullPointerException("input argument null");
    }
    int compare = Integer.compare(getPort(), o.getPort());
    if (compare == 0) {
      compare = getHostname().compareTo(o.getHostname());
    }
    return compare;
  }
Return a list of {@link DiskId}s from this data node.
@return
ambry-api/src/main/java/com/github/ambry/clustermap/DataNodeId.java