title: Spark cache disk VS cache memory
body: <p><a href="https://i.stack.imgur.com/VItfn.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/VItfn.png" alt="enter image description here"></a>
   I have a question about the follow picture.I use <i>data.persist(StorageLevel.MEMORY_AND_DISK_SER)</i>
  to cache our original data,but what is so surprised is that the speed we cached in memory is the same as the speed we cached in disk?why?I feel the speed we cached in memory should be faster than the speed we cached in disk,who can help me with this problem?</p>