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.