service/forum.md

@@ -195,6 +195,18 @@ command-line:
      git pull
      ./launcher rebuild app
 
+#### PostgreSQL upgrade not working
+
+The upgrade script may not succeed when upgrading to a newer version of
+PostgreSQL, even though it reports success. In the upgrade log, this message
+is logged:
+
+     mv: cannot move '/shared/postgres_data' to '/shared/postgres_data_old': Device or resource busy
+
+This is caused by a particularity in our deployment because `postgres_data` is
+a mount point, so attempts to move the directory fails. The workaround is to
+apply [this patch](https://github.com/discourse/discourse_docker/pull/989).
+
 ## Disaster recovery
 
 In case the machine is lost, it's possible to restore the forum from backups.