Upgrade note for concurrency mode default change

This is a follow up for the release notes added in the commit
35207ee8b5 that changed the default mode
for the scheduler and the API services. At that time we missed to note
the upgrade impact of such change. So this patch extends the reno with
an upgrade note.

Change-Id: I280e7eb9c1da6eeaf50e96e8b19e296961f2651a
Signed-off-by: Balazs Gibizer <gibi@redhat.com>

doc/source/admin/concurrency.rst

@@ -24,7 +24,7 @@ Selecting concurrency mode for a service
 Since nova 33.0.0 (2026.1 Gazpacho) the nova-scheduler, nova-api, and
 nova-metadata are using native threading by default. The rest of the
 services are using eventlet by default in this release. The concurrency mode
-can be configured via setting the environment variable
+can be configured per service via setting the environment variable
 ``OS_NOVA_DISABLE_EVENTLET_PATCHING``. Setting that variable to ``true``
 requests the native threading mode while setting it to ``false`` requests the
 eventlet mode. If the variable is not set the above default is applied.
@@ -37,6 +37,10 @@ eventlet mode. If the variable is not set the above default is applied.
    Since nova 33.0.0 (2026.1 Gazpacho) also the nova-conductor can be switched
    to native threading mode.
 
+   Since nova 33.0.0 (2026.1. Gazpacho) the nova-scheduler, nova-metadata, and
+   nova-api using native threading mode by default but still can be switched
+   back to eventlet if needed.
+
 
 Tunables for the native threading mode
 --------------------------------------
@@ -114,3 +118,14 @@ adding the `read_timeout`__ connection parameter to the connection string.
 
 We recommend using both in deployments where Nova services are running in
 native threading mode.
+
+Upgrading to nova 33.0.0 (2026.1. Gazpacho) or newer
+----------------------------------------------------
+
+In nova 33.0.0 (2026.1. Gazpacho) the default concurrency mode of
+nova-scheduler, nova-api, and nova-metadata service are switched to native
+threading. We recommend to decouple the upgrade from the concurrency mode
+change to reduce the risk of issues. To do that either test and tune the native
+threading mode of these services already in 2025.2 (Flamingo) or ensure that
+your service configuration is explicitly using the eventlet mode before you
+upgrade and only change to threading mode after the upgrade was successful.

releasenotes/notes/threading-by-default-sch-api-meta-d2534ce9c7b69d8a.yaml

@@ -3,7 +3,20 @@ features:
   - |
     The default concurrency mode is now switched from eventlet to native
     threading for nova-scheduler, nova-api, and nova-metadata services.
-    The concurrency mode can still be switched back to eventlet if needed.
-    Please read the
+    The concurrency mode can still be switched back to eventlet if needed
+    per service. Please read the
     `concurrency <https://docs.openstack.org/nova/latest/admin/concurrency.html>`__
     guide for more details.
+upgrade:
+  - |
+    When you are upgrading to 2026.1 (Gazpacho) or newer the default
+    configuration of nova-scheduler, nova-api, and nova-metadata services
+    change to run these services with native threading mode by default
+    instead of the legacy eventlet mode. We recommend to decouple the
+    upgrade from the concurrency mode change to reduce the risk of issues.
+    To do that either test the native threading mode of these services already
+    in 2025.2 (Flamingo) or ensure that your service configuration is
+    explicitly using the eventlet mode before you upgrade and only change to
+    threading mode after the upgrade was successful. Please read the
+    `concurrency <https://docs.openstack.org/nova/latest/admin/concurrency.html>`__
+    guide for more details on how to configure the mode.