FSVolumeImpl.initializeCacheExecutor calls Guava's ThreadPoolExecutorBuilder. setNameFormat, passing in the String representation of the parent File. Guava will take the String whole and pass it to String.format, which uses % as a special character. That means that if parent.toString() contains a percentage sign, followed by a character that's illegal to use as a formatter in String.format(), you'll get an exception that stops the MiniCluster from starting up.
I did not check to see if this would also happen on a normal DataNode daemon.
initializeCacheExecutor should escape the parent file name before passing it in.