/**
  * @summary get the mapping from tier alias to the used bytes of the tier, the keys are in the
  *     order from tier aliases with smaller ordinals to those with larger ones.
  * @return the response object
  */
 @GET
 @Path(GET_USED_BYTES_ON_TIERS)
 @ReturnType("java.util.SortedMap<String, Long>")
 public Response getUsedBytesOnTiers() {
   SortedMap<String, Long> usedBytesOnTiers = new TreeMap<>(getTierAliasComparator());
   for (Map.Entry<String, Long> tierBytes : mStoreMeta.getUsedBytesOnTiers().entrySet()) {
     usedBytesOnTiers.put(tierBytes.getKey(), tierBytes.getValue());
   }
   return RestUtils.createResponse(usedBytesOnTiers);
 }