code: public synchronized List<OutputStream> getCheckpointImageOutputStreams(
        long imageTxId) throws IOException {
      List<OutputStream> list = new ArrayList<OutputStream>();
      for (ImageManager im : imageManagers) {
        list.add(im.getCheckpointOutputStream(imageTxId));
      }
      return list;
    }
docs: Get the list of output streams for all underlying image managers, given the
  checkpoint transaction id.