title: Where does Spark actually persist RDDs on disk?
body: <p>I am using persist on different storage levels, but I found no difference on performance when I was using <code>MEMORY_ONLY</code> and <code>DISK_ONLY</code>. </p>
  
  <p>I think there might be something wrong with my code... Where can I find the persisted RDDs on disk so that I can make sure they were actually persisted? </p>