Question about Serialization in Storage Level

[Jiang, Zhipeng]

Hi there,

This question may seem to be kind of naïve, but what's the difference between 
MEMORY_AND_DISK and MEMORY_AND_DISK_SER?

If I call rdd.persist(StorageLevel.MEMORY_AND_DISK), the BlockManager won't 
serialize the rdd?

Thanks,
Zhipeng