In a preferred embodiment of the present invention, the replication horizon and other data propagation-related information is tracked through the use of a \u201cpulse\u201d and \u201cecho\u201d approach. Other embodiments may be described. Acutely angled ramp structures may be provided on both top and bottom surfaces of optical devices for distributing light along such top surface. This alloy material has the L10 structure, and is -oriented. A hub cap may be attachable to the wheel on a first side of the wheel and include mounting extensions for attachment of the hub cap to the threaded holes around hub.