Add support for async IO verification offload

This adds support for setting up a number of IO verification offload
threads, instead of doing the offload inline. An option for controlling
the CPU affinity of those threads are always added.

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
diff --git a/HOWTO b/HOWTO
index 708eca0..55662d3 100644
--- a/HOWTO
+++ b/HOWTO
@@ -827,6 +827,16 @@
 		before quitting on a block verification failure. If this
 		option is set, fio will exit the job on the first observed
 		failure.
+
+verify_async=int	Fio will normally verify IO inline from the submitting
+		thread. This option takes an integer describing how many
+		async offload threads to create for IO verification instead,
+		causing fio to offload the duty of verifying IO contents
+		to one or more separate threads.
+
+verify_async_cpus=str	Tell fio to set the given CPU affinity on the
+		async IO verification threads. See cpus_allowed for the
+		format used.
 		
 stonewall	Wait for preceeding jobs in the job file to exit, before
 		starting this one. Can be used to insert serialization