code: private static int getTierRankValue(String input) {
      // MEM, SSD, and HDD are the most commonly used Alluxio tier alias,
      // so we want them to show before other tier names
      // MEM, SSD, and HDD are sorted according to the speed of access
      List<String> tierOrder = Arrays.asList("MEM", "SSD", "HDD");
      int rank = tierOrder.indexOf(input);
      if (rank == -1) {
        return Integer.MAX_VALUE;
      }
      return rank;
    }
docs: Assigns a rank value to the input string.
  
  @param input the input to turn to rank value
  @return a rank value used to sort tiers