Dell PowerEdge M420 8/4 Gbps FC SAN Module Administrator's Guide - Page 32

3 Failover Failover Port Grouping policy considerations Following are the considerations for the Port Grouping policy: • A port cannot be a member of more than one port group. • The PG policy is enabled by default. A default port group "0" (PG0) is created, which contains all ports on the FC SAN Module. • APC policy and PG policy are mutually exclusive. You cannot enable these policies at the same time. • If an external port (N_Port) is added to a port group or deleted from a port group and login balancing is enabled or disabled for the port group, the external port maintains its original failover or failback setting. If an external port is deleted from a port group, it automatically gets added to port group 0. • When specifying a preferred secondary external port (N_Port) for a port group, the second port must be from the same group. If you specify an external port as a preferred secondary port and it already belongs to another port group, the operation fails. Therefore, it is recommended to form groups before defining the preferred secondary path. • If the PG policy is disabled while the FC SAN Module is online, all the defined port groups are deleted, but the port mapping remains unchanged. Before disabling the PG policy, you should save the configuration using the configupload command in case you might need this configuration again. • If external ports (N_Ports) connected to unrelated fabrics are grouped together, external port failover within a port group can cause the internal ports (F_Ports) to connect to a different fabric and the ports may lose connectivity to the targets they were connected to before the failover, thus causing I/O disruption as shown in Figure 4 on page 13. Ensure that the port group mode is set to Managed Fabric Name Monitoring (MFNM) mode. This monitors the port group to detect connection to multiple fabrics and disables failover of the external ports in the port group. For more information on MFNM, refer to "Enabling Managed Fabric Name Monitoring mode" on page 17. Failover ensures maximum uptime for the servers. Failover is enabled by default and is enforced during power-up. Failover allows internal ports (F_Ports) to automatically remap to an online external port (N_Port) if the primary external port goes offline. If multiple external ports are available for failover, failover evenly distributes the internal ports to available internal ports belonging to the same external port group. If no other external is available, failover does not occur. The Dell FC SAN Module provides an option to specify a secondary failover external port for an internal port. This external port is called the preferred secondary port for failover. If you specify a preferred secondary external port for any of the internal ports, and if the primary mapped external port goes offline, the internal ports will fail over to the preferred secondary external port (if it is online), then re-enable. The preferred secondary external port (N_Port) that you specify must be online; otherwise, the internal ports (F_Ports) will become disabled. The preferred secondary port is defined per internal port. For example, if two internal ports are mapped to a primary external port 1, you can define a secondary port for one of those internal ports and not define a secondary port for the other internal port. Refer to "Adding a preferred secondary external port (N_Port)" on page 21 for more information. 18 Dell 8/4Gbps FC SAN Module Administrator's Guide 53-1001345-01