diff --git a/cinder/cmd/manage.py b/cinder/cmd/manage.py
index 5678fc07a04..f401b1ab77c 100644
--- a/cinder/cmd/manage.py
+++ b/cinder/cmd/manage.py
@@ -259,6 +259,9 @@ class HostCommands(object):
 class DbCommands(object):
     """Class for managing the database."""
 
+    # NOTE: Online migrations cannot depend on having Cinder services running.
+    # Migrations can be called during Fast-Forward Upgrades without having any
+    # Cinder services up.
     online_migrations = (
         # Added in Queens
         db.service_uuids_online_data_migration,