mrg_buff_doc: Add documentation on mergable buffer option

Adds section about mergable buffers option to testusage doc.

Also adds drive boot type info into gotchas section of
testusage doc.

JIRA: VSPERF-459

Change-Id: I159273502663dd29f4847b0737535009a42e430d
Signed-off-by: Christian Trautman <ctrautma@redhat.com>

1 files changed, 38 insertions, 0 deletions

diff --git a/docs/userguide/testusage.rst b/docs/userguide/testusage.rst
index 46413f03..adbe603b 100755
--- a/docs/userguide/testusage.rst
+++ b/docs/userguide/testusage.rst
@@ -487,6 +487,31 @@ multiple VM NIC pairs.
 **NOTE:** In case of linux_bridge, all guest NICs are connected to the same
 bridge inside the guest.
 
+Mergable Buffers Options with QEMU
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+Mergable buffers can be disabled with VSPerf within QEMU. This option can
+increase performance significantly when not using jumbo frame sized packets.
+By default VSPerf disables mergable buffers. If you wish to enable it you
+can modify the setting in the a custom conf file.
+
+.. code-block:: python
+
+    GUEST_NIC_MERGE_BUFFERS_DISABLE = [False]
+
+Then execute using the custom conf file.
+
+.. code-block:: console
+
+        $ ./vsperf --conf-file=<path_to_custom_conf>/10_custom.conf
+
+Alternatively you can just pass the param during execution.
+
+.. code-block:: console
+
+        $ ./vsperf --test-params "GUEST_NIC_MERGE_BUFFERS_DISABLE=[False]"
+
+
 Selection of dpdk binding driver for tests with VMs
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
@@ -716,6 +741,19 @@ Example of manual pylint invocation:
 GOTCHAs:
 ^^^^^^^^
 
+Custom image fails to boot
+~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Using custom VM images may not boot within VSPerf pxp testing because of
+the drive boot and shared type which could be caused by a missing scsi
+driver inside the image. In case of issues you can try changing the drive
+boot type to ide.
+
+.. code-block:: python
+
+    GUEST_BOOT_DRIVE_TYPE = ['ide']
+    GUEST_SHARED_DRIVE_TYPE = ['ide']
+
 OVS with DPDK and QEMU
 ~~~~~~~~~~~~~~~~~~~~~~~