Renaming columns requires downtime

db/migrate/20161103171205_rename_repository_storage_column.rb

@@ -5,12 +5,12 @@ class RenameRepositoryStorageColumn < ActiveRecord::Migration
   include Gitlab::Database::MigrationHelpers
 
   # Set this constant to true if this migration requires downtime.
-  DOWNTIME = false
+  DOWNTIME = true
 
   # When a migration requires downtime you **must** uncomment the following
   # constant and define a short and easy to understand explanation as to why the
   # migration requires downtime.
-  # DOWNTIME_REASON = ''
+  DOWNTIME_REASON = 'Renaming the application_settings.repository_storage column'
 
   # When using the methods "add_concurrent_index" or "add_column_with_default"
   # you must disable the use of transactions as these methods can not run in an

doc/development/what_requires_downtime.md

@@ -66,6 +66,12 @@ producing errors whenever it tries to use the `dummy` column.
 As a result of the above downtime _is_ required when removing a column, even
 when using PostgreSQL.
 
+## Renaming Columns
+
+Renaming columns requires downtime as running GitLab instances will continue
+using the old column name until a new version is deployed. This can result
+in the instance producing errors, which in turn can impact the user experience.
+
 ## Changing Column Constraints
 
 Generally changing column constraints requires checking all rows in the table to