Ceph » rgw

Radosgw creates a zone and zonegroup by default, but does not create a default realm. The default zone and zonegroup are both created with an empty "realm_id" field.

If a realm is later created, the 'radosgw-admin period update' command will reject the default zone and zonegroup because their "realm_id" does not match the realm's. This makes it difficult to add a multisite configuration to a cluster with existing data in the "default" pools. You have to manually edit the zone's json format to either: a) change the default zone's realm_id, or b) change a new zone's pool names to match the existing "default.rgw.*" pools.

My suggestion would be to create a realm by default and associate it with the default zone and zonegroup, so that realm could be extended later to include more zones or zonegroups.