Async verify HOWTO/man update

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
diff --git a/HOWTO b/HOWTO
index 55662d3..5099c83 100644
--- a/HOWTO
+++ b/HOWTO
@@ -832,7 +832,10 @@
 		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.
+		to one or more separate threads. If using this offload
+		option, even sync IO engines can benefit from using an
+		iodepth setting higher than 1, as it allows them to have
+		IO in flight while verifies are running.
 
 verify_async_cpus=str	Tell fio to set the given CPU affinity on the
 		async IO verification threads. See cpus_allowed for the
diff --git a/fio.1 b/fio.1
index 7b1fc80..fa54763 100644
--- a/fio.1
+++ b/fio.1
@@ -602,7 +602,9 @@
 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.
+to one or more separate threads.  If using this offload option, even sync IO
+engines can benefit from using an \fBiodepth\fR setting higher than 1, as it
+allows them to have IO in flight while verifies are running.
 .TP
 .BI verify_async_cpus \fR=\fPstr
 Tell fio to set the given CPU affinity on the async IO verification threads.