Document the wonders of scale testing nova with the fake driver

Devstack has some little known variables for running nova
with the fake compute driver and running several nova-compute
services on a single host, which can be useful for testing
move operations and scale testing of controller services like
nova-api and nova-scheduler.

This adds documentation about the fake virt driver and related
variables and scaling considerations when using them.

Change-Id: Ic89d463d0f3f180b323edd6e2c8ff0404638ef07

Author: mriedem

doc/source/guides/nova.rst

@@ -68,3 +68,69 @@ These config options are defined in `nova.conf.serial_console
 For more information on OpenStack configuration see the `OpenStack
 Compute Service Configuration Reference
 <https://docs.openstack.org/nova/latest/admin/configuration/index.html>`_
+
+
+Fake virt driver
+================
+
+Nova has a `fake virt driver`_ which can be used for scale testing the control
+plane services or testing "move" operations between fake compute nodes, for
+example cold/live migration, evacuate and unshelve.
+
+The fake virt driver does not communicate with any hypervisor, it just reports
+some fake resource inventory values and keeps track of the state of the
+"guests" created, moved and deleted. It is not feature-complete with the
+compute API but is good enough for most API testing, and is also used within
+the nova functional tests themselves so is fairly robust.
+
+.. _fake virt driver: http://git.openstack.org/cgit/openstack/nova/tree/nova/virt/fake.py
+
+Configuration
+-------------
+
+Set the following in your devstack ``local.conf``:
+
+.. code-block:: ini
+
+  [[local|localrc]]
+  VIRT_DRIVER=fake
+  NUMBER_FAKE_NOVA_COMPUTE=<number>
+
+The ``NUMBER_FAKE_NOVA_COMPUTE`` variable controls the number of fake
+``nova-compute`` services to run and defaults to 1.
+
+When ``VIRT_DRIVER=fake`` is used, devstack will disable quota checking in
+nova and neutron automatically. However, other services, like cinder, will
+still enforce quota limits by default.
+
+Scaling
+-------
+
+The actual value to use for ``NUMBER_FAKE_NOVA_COMPUTE`` depends on factors
+such as:
+
+* The size of the host (physical or virtualized) on which devstack is running.
+* The number of API workers. By default, devstack will run ``max($nproc/2, 2)``
+  workers per API service. If you are running several fake compute services on
+  a single host, then consider setting ``API_WORKERS=1`` in ``local.conf``.
+
+In addition, while quota will be disabled in neutron, there is no fake ML2
+backend for neutron so creating fake VMs will still result in real ports being
+created. To create servers without networking, you can specify ``--nic=none``
+when creating the server, for example:
+
+.. code-block:: shell
+
+  $ openstack --os-compute-api-version 2.37 server create --flavor cirros256 \
+      --image cirros-0.3.5-x86_64-disk --nic none --wait test-server
+
+.. note:: ``--os-compute-api-version`` greater than or equal to 2.37 is
+          required to use ``--nic=none``.
+
+To avoid overhead from other services which you may not need, disable them in
+your ``local.conf``, for example:
+
+.. code-block:: ini
+
+  disable_service horizon
+  disable_service tempest