Sync HOWTO iodepth option with man page

[fio.git] / HOWTO

diff --git a/HOWTO b/HOWTO
index 7cb24dd8c12e764a7ff5c5c5c813efc775418311..dc21afc039d36456e41ea215b4c2917274ecc74f 100644 (file)
--- a/HOWTO
+++ b/HOWTO
@@ -549,7 +549,14 @@ ioengine=str       Defines how the job issues io to the file. The following
 iodepth=int    This defines how many io units to keep in flight against
                the file. The default is 1 for each file defined in this
                job, can be overridden with a larger value for higher
-               concurrency.
+               concurrency. Note that increasing iodepth beyond 1 will not
+               affect synchronous ioengines (except for small degress when
+               verify_async is in use). Even async engines my impose OS
+               restrictions causing the desired depth not to be achieved.
+               This may happen on Linux when using libaio and not setting
+               direct=1, since buffered IO is not async on that OS. Keep an
+               eye on the IO depth distribution in the fio output to verify
+               that the achieved depth is as expected. Default: 1.
 
 iodepth_batch_submit=int
 iodepth_batch=int This defines how many pieces of IO to submit at once.