Ds400 Dual Controller Direct Attachment Dual Path - IBM TotalStorage DS300 Best Practices Manual

Figure 8-3 DS400 single path attachment

8.3.2 DS400 dual controller direct attachment dual path

In small environments it is possible to directly attach up to two host systems,
each with two HBAs, with two paths to the DS400. It is important that both hosts
be attached to both controllers to be protected against a controller failure. A
failover driver is required for this situation. The logical must be assigned to both
initiators.
When no failover driver is available for the host operating system, each logical
drive must be assigned only to a single initiator. This is a configuration with two
single paths.
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IBM TotalStorage DS300 and DS400 Best Practices Guide