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distributed across two host ports and managed by both controllers.

Figure 10 - 6 Mapping System Drives (IDs)

!Some operating systems do not read multiple LUNs under single ID. As diagrammed above, you may have the host channel to present several IDs and map logical configurations to these IDs. Each of these IDs can be identified as Primary or Secondary. As a rule for most operation systems, each configuration unit will be mapped to LUN0 under each ID.

10.1.4 Fault-Tolerance

What Is a Redundant Controller Configuration?

Hardware failures can occur. A simple parity error may sometimes cause a RAID system to completely hang up. Having two controllers working together will guarantee that at least one controller will survive the catastrophes and keep the system working. This is the logic behind having the redundant controllers – to minimize the best we could the chance of down time for a storage subsystem.

A redundant controller system uses two controllers to manage the storage arrays. It requires two controllers to work together and both must be working normally. During normal operation, each controller serves its I/O requests. If one controller should fail, the existing controller will temporarily take over for the failed controller until it is replaced. The failover and failback processes should be totally transparent to host and require only minimum efforts to restore the original configuration.

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