a to determine the presence of the secondary therefore fig1 illustrates a computing environment 100 in which a fibre channel adapter 108 a coupled to a primary storage control unit 102 sends a pdisc extended link service frame to determine the presence of the secondary storage control unit 104 the pdisc extended link service frame is non disruptive to i o operations occurring between the primary storage control unit 102 and the secondary storage control unit 104 fig2 illustrates a block diagram of data structures implemented in the storage control units 102 104 106 and the fibre channel adapters 108 a 108 q 110 a 110 r 112 a 112 s in accordance with certain described implementations of the invention a storage control unit 200 where the storage control unit 200 may be any of the storage control units 102 104 106 may include a data structure that corresponds to a world wide node name wwnn 202 where the wwnn 202 is an identification of the storage control unit 200 a fibre channel adapter 204 where the fibre channel adapter 204 may be any of the fibre channel adapters 108 a 108 q 110 a 110 r 112 a 112 s may include a port 206 the fibre channel adapter 204 may communicate.