code: String getDeprecatedProperty(String prop) {
      assert getLayoutVersion() > FSConstants.LAYOUT_VERSION :
        "getDeprecatedProperty should only be done when loading " +
        "storage from past versions during upgrade.";
      return deprecatedProperties.get(prop);
    }
docs: Return a property that was stored in an earlier version of HDFS.
  
  This should only be used during upgrades.