Here, you see the importance that the number of buffers has in overall performance. You need  enough buffers to ensure that the transmitting port can continue to send frames without  stopping to use the full bandwidth, which is true with distance. The total amount of buffer  credit needed to optimize the throughput depends on the link speed and the average  frame size. For example, consider an 8 Gbps link connecting two switches that are 100 km apart. At  8 Gbps, a full frame (2148 bytes) occupies about 0.51 km of fiber. In a 100 km link, you can  send 198 frames before the first one reaches its destination. You need an ACK to go back to  the start to fill EE_Credit again. You can send another 198 frames before you receive the  first ACK.  You need at least 396 buffers to allow for nonstop transmission at 100 km distance. The  maximum distance that can be achieved at full performance depends on the capabilities of  the FC node that is attached at either end of the link extenders, which are vendor-specific. A  match should occur between the buffer credit capability of the nodes at either end of the  extenders.  Ports and zoning requirements for the remote system partnership have changed over time.  The current preferred configuration is based on the following: Flash Alert The preferred practice for the IBM Spectrum Virtualize based systems is to provision  dedicated node ports for local node-to-node traffic (by using port masking) and isolate Global  Mirror node-to-node traffic between the local nodes from other local SAN traffic.  Remote port masking:  To isolate the node-to-node traffic from the remote copy traffic, the  local and remote port masking implementation is preferable. This configuration of local node port masking is less of a requirement on non-clustered  FS9100 systems, where traffic between node canisters in an I/O group is serviced by the  dedicated PCI inter-canister link in the enclosure. The following guidelines apply to the  remote system connectivity: This replication data transfer occurs through the node-to-node connectivity. Note that this  configuration, even though supported, allows the replication traffic to go through the  node-to-node connectivity and this is not recommended. GLYPH<SM590000> Where possible, use the minimum number of partnerships between systems. For example,  assume site A contains systems A1 and A2, and site B contains systems B1 and B2. In  this scenario, creating separate partnerships between pairs of systems (such as A1-B1  and A2-B2) offers greater performance for Global Mirror replication between sites than a  configuration with partnerships defined between all four systems. GLYPH<SM590000> For maximum throughput, all nodes in each system should be used for replication, both in  terms of balancing the preferred node assignment for volumes and for providing  intersystem Fibre Channel connectivity. GLYPH<SM590000> Partnered systems should use the same number of nodes in each system for replication. GLYPH<SM590000> The minimum requirement to establish a remote copy partnership is to connect at least  one node per system. When remote connectivity among all the nodes of both systems is  not available, the nodes of the local system not participating to the remote partnership will  use the node/nodes defined in the partnership as a bridge to transfer the replication data  to the remote system.  Remote system ports and zoning considerations Chapter 6. Copy services  187