IBM TotalStorage DS300 Best Practices Manual page 458

We describe three example configurations of a DS400 dual controller fabric
attachment:
Fabric attachment with dual paths
Fabric attachment with quad paths through interswitch link
Fabric attachment with quad paths without interswitch link
Fabric attachment with dual paths
A very straightforward configuration of a fabric attachment with dual paths is
shown in Figure 8-5 on page 441. Each host system has two paths to the DS400.
One path is from the first HBA of the host system to controller A of the DS400.
The other path is from the second HBA of the host system to controller B of the
DS400.
An outage of an HBA or its connection to the SAN switch leads to a controller
failover for an array if the active path ran over this HBA. The other configurations
with quad paths cope with such an outage and can use a second path to the
same controller, preventing a controller failover.
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IBM TotalStorage DS300 and DS400 Best Practices Guide