 /*
   * Roll back to old snapshot at the block pool level
   * If previous directory exists: 
   * <ol>
   * <li>Rename <SD>/current/<bpid>/current to removed.tmp</li>
   * <li>Rename * <SD>/current/<bpid>/previous to current</li>
   * <li>Remove removed.tmp</li>
   * </ol>
   * 
   * Do nothing if previous directory does not exist.
   * @param bpSd Block pool storage directory at <SD>/current/<bpid>
   */
 /*
   * Roll back to old snapshot at the block pool level
   * If previous directory exists: 
   * <ol>
   * <li>Rename <SD>/current/<bpid>/current to removed.tmp</li>
   * <li>Rename * <SD>/current/<bpid>/previous to current</li>
   * <li>Remove removed.tmp</li>
   * </ol>
   * 
   * Do nothing if previous directory does not exist.
   * @param bpSd Block pool storage directory at <SD>/current/<bpid>
   */ | BlockPoolSliceStorage.java