Add iodepth_batch setting

This allows controlling how much IO we submit in one go
independent of the iodepth set.

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
diff --git a/HOWTO b/HOWTO
index 237b3ad..de4b0a9 100644
--- a/HOWTO
+++ b/HOWTO
@@ -294,6 +294,10 @@
 		job, can be overridden with a larger value for higher
 		concurrency.
 
+iodepth_batch=int This defines how many pieces of IO to submit at once.
+		It defaults to the same as iodepth, but can be set lower
+		if one so desires.
+
 iodepth_low=int	The low water mark indicating when to start filling
 		the queue again. Defaults to the same as iodepth, meaning
 		that fio will attempt to keep the queue full at all times.