Remove use of nomodeset by default

The troubleshooting kernel command line option nomodeset
unfortunately changes the way framebuffer interactions work
with graphics devices which in some cases can result in kernel
memory to be used for graphics updates. When this happens on
some specific hardware common in rack mount servers with baseboard
management controllers, this can cause the memory bus to become
locked for a brief time while the graphics update is occuring.

This locked memory bus means disk IO can become blocked,
and network cards can overflow their buffers resulting in
packet loss on top of the latency incurred by the graphics
update executing.

As such, we've removed the nomodeset option from default usage and
added a note describing its removal to the documentation along
with a release note.

Change-Id: I9084d88c3ec6f13bd64b8707892758fa87dd7f86

1 files changed, 12 insertions, 4 deletions

diff --git a/doc/source/admin/interfaces/boot.rst b/doc/source/admin/interfaces/boot.rst
index b9f0bda3e..a4b4cf2a3 100644
--- a/doc/source/admin/interfaces/boot.rst
+++ b/doc/source/admin/interfaces/boot.rst
@@ -49,7 +49,7 @@ configuration option:
 .. code-block:: ini
 
     [pxe]
-    kernel_append_params = nofb nomodeset vga=normal
+    kernel_append_params = nofb vga=normal
 
 .. note::
    The option was called ``pxe_append_params`` before the Xena cycle.
@@ -59,9 +59,9 @@ Per-node and per-instance overrides are also possible, for example:
 .. code-block:: bash
 
   baremetal node set node-0 \
-    --driver-info kernel_append_params="nofb nomodeset vga=normal"
+    --driver-info kernel_append_params="nofb vga=normal"
   baremetal node set node-0 \
-    --instance-info kernel_append_params="nofb nomodeset vga=normal"
+    --instance-info kernel_append_params="nofb vga=normal"
 
 Starting with the Zed cycle, you can combine the parameters from the
 configuration and from the node using the special ``%default%`` syntax:
@@ -74,13 +74,21 @@ configuration and from the node using the special ``%default%`` syntax:
 Together with the configuration above, the following parameters will be
 appended to the kernel command line::
 
-    nofb nomodeset vga=normal console=ttyS0,115200n8
+    nofb vga=normal console=ttyS0,115200n8
 
 .. note::
    Ironic does not do any de-duplication of the resulting kernel parameters.
    Both kernel itself and dracut seem to give priority to the last instance
    of the same parameter.
 
+.. warning::
+   Previously our documentation listed the Linux kernel parameter
+   ``nomodeset`` as an option. This option is intended for troubleshooting,
+   and can greatly degrade performance with Matrox/Aspeed BMC Graphics
+   controllers which is very commonly used on physical servers. The
+   performance degredation can greatly reduce IO capacity upon every
+   console graphics update being written to the screen.
+
 Common options
 --------------