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, 22 insertions, 0 deletions

diff --git a/releasenotes/notes/remove-nomodset-7a352a9519c1045b.yaml b/releasenotes/notes/remove-nomodset-7a352a9519c1045b.yaml
new file mode 100644
index 000000000..26d10c262
--- /dev/null
+++ b/releasenotes/notes/remove-nomodset-7a352a9519c1045b.yaml
@@ -0,0 +1,22 @@
+---
+upgrade:
+  - |
+    The defaults for ``kernel_append_params`` have had the Linux kernel
+    command line parameter ``nomodeset`` removed from the defaults for the
+    ``kernel_append_params`` settings. The ``nomodeset`` option is for
+    troubleshooting and changes the behavior of the graphics interface
+    such that memory can be locked upon graphical updates on physical
+    servers with BMC graphical interfaces, which results in spikes in
+    latency and packet loss whenever graphics updates occur. Operators
+    may add the option to their local configuration, but should be aware
+    that large image transfers or other high IO operations can be impacted.
+fixes:
+  - |
+    Agents deploying on physical servers with default kernel arguments were
+    suspetible to packet loss if a Matrox VGA/Aspeed BMC Graphics interface
+    is present on the machine. The defaults have been changed to remove
+    the use of the ``nomodeset`` kernel command line parameter which
+    should only be used for troubleshooting as it has been determined
+    that the memory updates can lock all of the kernel memory upon any
+    console graphics update which can negatively impact IO for Networking
+    or Disk interactions.