[PATCH] fio: warn about "ioengine=psync" and "iodepth >= 1"

[Alexey Dobriyan <adobriyan@xxxxxxxxx>]

Some users are using both ioengine=psync and iodepth=2+
without blinking in front of the millions:

	https://youtu.be/coShLkCriXc?t=840
	https://youtu.be/coShLkCriXc?t=890
	
Help them with helpful message.

Signed-off-by: Alexey Dobriyan <adobriyan@xxxxxxxxx>
---

 backend.c |    5 +++++
 1 file changed, 5 insertions(+)

--- a/backend.c
+++ b/backend.c
@@ -1791,6 +1791,11 @@ static void *thread_main(void *data)
 	if (td_io_init(td))
 		goto err;
 
+	if (td_ioengine_flagged(td, FIO_SYNCIO) && td->o.iodepth > 1) {
+		log_info("note: both iodepth>=1 and synchronous I/O engine are selected, "
+			 "queue depth will be capped at 1\n");
+	}
+
 	if (init_io_u(td))
 		goto err;