code: public File[] getFsImageNameCheckpoint(long txid) {
      ArrayList<File> list = new ArrayList<File>();
      for (Iterator<StorageDirectory> it =
                   dirIterator(NameNodeDirType.IMAGE); it.hasNext();) {
        list.add(getStorageFile(it.next(), NameNodeFile.IMAGE_NEW, txid));
      }
      return list.toArray(new File[list.size()]);
    }
docs: Return the name of the image file that is uploaded by periodic
  checkpointing
  
  @return List of filenames to save checkpoints to.