Add man page and HOWTO for io_submit_mode option

[fio.git] / HOWTO

diff --git a/HOWTO b/HOWTO
index 60eab243caadf8c473166c0d56816adb5ffbc397..bcc72b5882be95e4804a641f151cde13b214b432 100644 (file)
--- a/HOWTO
+++ b/HOWTO
@@ -820,6 +820,18 @@ iodepth_low=int    The low water mark indicating when to start filling
                after fio has filled the queue of 16 requests, it will let
                the depth drain down to 4 before starting to fill it again.
 
+io_submit_mode=str     This option controls how fio submits the IO to
+               the IO engine. The default is 'inline', which means that the
+               fio job threads submit and reap IO directly. If set to
+               'offload', the job threads will offload IO submission to a
+               dedicated pool of IO threads. This requires some coordination
+               and thus has a bit of extra overhead, especially for lower
+               queue depth IO where it can increase latencies. The benefit
+               is that fio can manage submission rates independently of
+               the device completion rates. This avoids skewed latency
+               reporting if IO gets back up on the device side (the
+               coordinated omission problem).
+
 direct=bool    If value is true, use non-buffered io. This is usually
                O_DIRECT. Note that ZFS on Solaris doesn't support direct io.
                On Windows the synchronous ioengines don't support direct io.