code: public synchronized void saveDigestAndRenameCheckpointImage(long txid,
        MD5Hash digest) throws IOException {
      for (ImageManager im : imageManagers) {
        if (im.saveDigestAndRenameCheckpointImage(txid, digest)) {
          // restore enabled state
          im.setImageDisabled(false);
        } else {
          // failed image
          im.setImageDisabled(true);
        }
      }
      checkImageManagers();
    }
docs: Save digest in all underlying image managers, and rename the checkpoint
  file. This will throw an exception if all image managers fail.