static void saveAuIdProperties(String location, Properties props) {
   // XXX these AU_ID_FILE entries need to be backed up elsewhere to avoid
   // single-point corruption
   File propDir = new File(location);
   if (!propDir.exists()) {
     logger.debug("Creating directory '" + propDir.getAbsolutePath() + "'");
     propDir.mkdirs();
   }
   File propFile = new File(propDir, AU_ID_FILE);
   try {
     logger.debug3("Saving au id properties at '" + location + "'.");
     OutputStream os = new BufferedOutputStream(new FileOutputStream(propFile));
     props.store(os, "ArchivalUnit id info");
     os.close();
     propFile.setReadOnly();
   } catch (IOException ioe) {
     logger.error("Couldn't write properties for " + propFile.getPath() + ".", ioe);
     throw new LockssRepository.RepositoryStateException("Couldn't write au id properties file.");
   }
 }