/**
  * @summary get the mapping from tier alias to the paths of the directories in the tier
  * @return the response object
  */
 @GET
 @Path(GET_DIRECTORY_PATHS_ON_TIERS)
 @ReturnType("java.util.SortedMap<String, java.util.List<String>>")
 public Response getDirectoryPathsOnTiers() {
   SortedMap<String, List<String>> tierToDirPaths = new TreeMap<>(getTierAliasComparator());
   tierToDirPaths.putAll(mStoreMeta.getDirectoryPathsOnTiers());
   return RestUtils.createResponse(tierToDirPaths);
 }