Use 512kb max readahead size, and 32kb min readahead size.
The former helps io performance for common workloads.
The latter will be used in the thrashing safe context readahead.
CC: Jens Axboe<jens.axboe@xxxxxxxxxx>
CC: Chris Mason<chris.mason@xxxxxxxxxx>
CC: Peter Zijlstra<a.p.zijlstra@xxxxxxxxx>
CC: Martin Schwidefsky<schwidefsky@xxxxxxxxxx>
CC: Paul Gortmaker<paul.gortmaker@xxxxxxxxxxxxx>
CC: Matt Mackall<mpm@xxxxxxxxxxx>
CC: David Woodhouse<dwmw2@xxxxxxxxxxxxx>
Tested-by: Vivek Goyal<vgoyal@xxxxxxxxxx>
Tested-by: Christian Ehrhardt<ehrhardt@xxxxxxxxxxxxxxxxxx>
Acked-by: Christian Ehrhardt<ehrhardt@xxxxxxxxxxxxxxxxxx>
Signed-off-by: Wu Fengguang<fengguang.wu@xxxxxxxxx>