code: public static String makeRelative(URI cache, Configuration conf)
      throws IOException {
      String host = cache.getHost();
      if (host == null) {
        host = cache.getScheme();
      }
      if (host == null) {
        URI defaultUri = FileSystem.get(conf).getUri();
        host = defaultUri.getHost();
        if (host == null) {
          host = defaultUri.getScheme();
        }
      }
      String path = host + cache.getPath();
      path = path.replace(":/","/");                // remove windows device colon
      return path;
    }
docs: /*
  Returns the relative path of the dir this cache will be localized in
  relative path that this cache will be localized in. For
  hdfs://hostname:port/absolute_path -- the relative path is
  hostname/absolute path -- if it is just /absolute_path -- then the
  relative path is hostname of DFS this mapred cluster is running
  on/absolute_path