Add time_based option

This allows fio to keep running, even if the workload has completed.
It will simply restart the workload over and over, for as long as the
runtime setting allows.

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
diff --git a/HOWTO b/HOWTO
index 2cc62b2..5fdc1f7 100644
--- a/HOWTO
+++ b/HOWTO
@@ -477,6 +477,11 @@
 		a specified job will run, so this parameter is handy to
 		cap the total runtime to a given time.
 
+time_based	If set, fio will run for the duration of the runtime
+		specified even if the file(s) are completey read or
+		written. It will simply loop over the same workload
+		as many times as the runtime allows.
+
 invalidate=bool	Invalidate the buffer/page cache parts for this file prior
 		to starting io. Defaults to true.