code: public void cleanDirectory() {
  		File[] directories = _parentDirectory.listFiles(new FilenameFilter() {
  			@Override
  			public boolean accept(File dir, String name) {
  				return name.startsWith(DIRECTORY_PREFIX);
  			}
  		});
  		for (File directory : directories) {
  			delete(directory);
  		}
  	}
docs: Cleans the parent directory of this storage provider. This action will
  delete all previous collection storages made in this directory, and thus
  it should only be invoked either before any collections has been made or
  when all collections are ensured to be unused.