2010/12/5 魏金仙 <sei_wjx@126.com>
for each sstable, there is an index file, which is loaded in memory to locate a particular key's offset efficiently.

Index files are not held in memory.
 
and for each CF, KeysCached can be set to cache keys' location.
could you pls tell me the difference between the two?

KeysCached are held in memory.
 
I'm wondering whether it's necessary to set "KeysCached" for a CF or not.

Not necessary, but advantageous. 

-Brandon