neutron: adds elements regardings neutron dhcp agent az aware

Change-Id: Ife47a45740c35e431a3d72709f6a91fce4c9dab7
Signed-off-by: Sahid Orentino Ferdjaoui <sahid.ferdjaoui@industrialdiscipline.com>

doc/source/journey/configure/index.rst

@@ -10,6 +10,7 @@ Contents:
    rabbitmq
    database
    other
+   neutron
 
 ---------
 Configure
@@ -25,6 +26,7 @@ Once your cluster is properly configured to handle scale, you can go to the seco
 Resources
 ---------
 
+* :doc:`Large Scale Configuration guidelines for Neutron </journey/configure/neutron>`
 * :doc:`Large Scale Configuration guidelines for RabbitMQ </journey/configure/rabbitmq>`
 * :doc:`Large Scale Configuration guidelines for Database </journey/configure/database>`
 * :doc:`Large Scale Configuration guidelines (other/extra) </journey/configure/other>`

doc/source/journey/configure/neutron.rst

@@ -0,0 +1,28 @@
+=======
+Neutron
+=======
+
+Contents:
+
+.. toctree::
+   :maxdepth: 1
+
+   neutron/neutron-dhcp-agents-azaware
+
+Introduction
+------------
+
+This document gathers information and references regarding large-scale
+configuration for Neutron.
+
+
+Neutron Agents
+--------------
+
+* :doc:`Neutron DHCP Agents AZ Awareness </journey/configure/neutron/neutron-dhcp-agents-azaware>`
+
+
+Stories
+-------
+
+* :doc:`Various latency issues from Société Générale </stories/2023-10-06>`

doc/source/journey/configure/neutron/neutron-dhcp-agents-azaware.rst

@@ -0,0 +1,66 @@
+Neutron DHCP Agents Availability Zone Awareness
+===============================================
+
+In large deployments requiring High Availability, it is crucial to ensure
+that the network agents and in particular for this document, the DHCP
+agent, run on different Availability Zones.
+
+Nova AZ is not Neutron AZ
+-------------------------
+
+Logically, an AZ is the same concept between Nova and Neutron, but
+functionally, they do not have any relation. This means that when you
+define AZ in Nova, it does not impact Neutron.
+
+By default, the `BaseWeightScheduler` schedules networks on DHCP
+Agents independently of the related AZ. Even if you set AZ hints for a
+network, the scheduler will select agents with the fewest networks to
+handle to fulfill the `dhcp_agents_per_network` requirement.
+
+For example, if you configure `dhcp_agents_per_network = 2` and deploy
+2 agents in AZ1 and 2 agents in AZ2, the scheduler may schedule the
+first network (net1) on both DHCP agents in AZ1 and the second network
+on those in AZ2.
+
+This situation is not desirable, as if something goes wrong in AZ1,
+network net1 can be unreachable.
+
+AZAwareWeightScheduler
+----------------------
+
+Neutron provides a solution to make the scheduler comprehend AZs
+better. To achieve this, configure Neutron to use
+`AZAwareWeightScheduler`.
+
+.. code-block:: ini
+
+   network_scheduler_driver = neutron.scheduler.dhcp_agent_scheduler.AZAwareWeightScheduler
+   dhcp_load_type = networks
+
+Now, the scheduler understands AZs.
+
+The next step is to ensure that all networks are scheduled on both
+AZs. Set the default_availability_zones to include both AZs.
+
+.. code-block:: ini
+
+   default_availability_zones = AZ1, AZ2
+
+It is also possible to add more granularity for a network by setting
+hints:
+
+For example, if you want a network to be scheduled in a specific zone,
+say AZ3:
+
+.. code-block:: bash
+
+   openstack network create --availability-zone-hint AZ3
+
+Conclusion
+-----------
+
+In conclusion, configuring Neutron to be AZ-aware for its agents,
+particularly the DHCP agent in the context of this document, is
+relatively straightforward. It requires minimal and non-excessive
+changes, yet it can significantly elevate your environment to a higher
+level of robustness and High Availability.