What:		/sys/block/<device>/iosched/target_latency
  Date:		March 2012
  contact:	Tao Ma <boyu.mt@taobao.com>
  Description:
  		The /sys/block/<device>/iosched/target_latency only exists
  		when the user sets cfq to /sys/block/<device>/scheduler.
  		It contains an estimated latency time for the cfq. cfq will
  		use it to calculate the time slice used for every task.