calculate cache hit rate correctly in global cache IO.
calculate cache hit rate correctly in global cache IO.

icoming/FlashGraph,icoming/FlashGraph,zheng-da/FlashX,icoming/FlashX,icoming/FlashX,icoming/FlashGraph,flashxio/FlashX,flashxio/FlashX,zheng-da/FlashX,flashxio/FlashX,icoming/FlashGraph,zheng-da/FlashX,icoming/FlashX,icoming/FlashGraph,flashxio/FlashX,icoming/FlashX,flashxio/FlashX,flashxio/FlashX,silky/FlashGraph,silky/FlashGraph,icoming/FlashX,silky/FlashGraph,zheng-da/FlashX,zheng-da/FlashX,silky/FlashGraph,silky/FlashGraph