IBM Midrange System DS4000 Series Hardware Manual page 437

Previously, there used to be a variety of host cable configuration rules and recommendations
for the different implementations of the RDAC multipath driver. In most cases, these were
restricted to a maximum of two paths. With MPIO and other native multipath drivers, there is a
single highly resilient recommended configuration for Dual-HBA hosts attached through
switched Fibre Channel SAN fabrics, as illustrated in Figure 7-62.
Server 1 Server 2
HBA 1 HBA 2 HBA 1 HBA 2
FC Switch FC Switch
Controller A Controller B
Storage Subsystem
Figure 7-62 HBA to storage recommended configuration
Direct attached host configurations are permitted, although check the readme files for any
operating system specific restrictions. Brocade HBAs do not support direct attached
connections.
Single HBA configurations are permitted through a switch. The HBA must be zoned to have
access to both controllers.
Logical drive not on preferred path
Each logical drive on the DS4000 or DS5000 storage subsystem is assigned to a preferred
controller. The owning controller will service all I/O requests for this logical drive, while the
alternate controller will act as a standby in case of failure. Whenever a logical drive failover
occurs, a critical event is logged in MEL and Recovery Guru reports a non-optimal condition
due to the logical drive not being on the preferred path. In the majority of cases, this does not
indicate a fault on the DS4000 or DS5000 storage subsystem, but simply that the controllers
behaved exactly as designed in transferring ownership when requested. Usually, the root
cause of the failover is external to DS4000 or DS5000 storage subsystem.
Selecting the correct host type definition
The DS4000 or DS5000 storage subsystem automatically moves a logical drive onto the
alternate controller for host types where Automatic Drive Transfer (ADT) is enabled whenever
an I/O is received down the non-preferred path. Otherwise, it relies on the host multipath
driver to issue a command (SCSI Mode Select 2C) to instruct the DS4000 or DS5000 storage
subsystem when a failover is required. For these mechanisms to work, it is essential that the
host type is set correctly to match the operating system and multipath driver on the host.
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Chapter 7. Advanced maintenance, troubleshooting, and diagnostics