Compaq 3000 Controller Features, Bridging the Gap Between the Host and Device Expansion Shelf

Chapter 2. RAID Array Controller

There are two configurations for redundant pairs of controllers: Active/Active Failover mode and Active/Passive Failover mode. In Active/Active Failover, each controller in the redundant pair has one active SCSI host port and one pas- sive SCSI host port. Redundancy Groups (Virtual LUNS) can be mapped only to one active host port and are not accessible from the passive port or the other controller (i.e. partitioned model).

In Active/Passive Failover, one controller in the redundant pair has both SCSI host ports active and the other controller is in a standby passive mode. Redun- dancy Groups (Virtual LUNs) can be mapped to either SCSI host port or to both as in the single controller model.

In both cases a single controller failure will not affect the subsystem because the surviving controller will take over.

2.2Controller Features

The controller is the intelligent bridge between the host and the devices in the shelf. From the host’s perspective, the controller is simply another SCSI device connected to one of its I/O buses. Consequently, the host sends its I/O requests to the controller just as it would to any other SCSI device. Figure 2–2 shows the role of the controller between the host and the Device Expansion Shelf.

Figure 2–2 Bridging the Gap Between the Host and Device Expansion Shelf

Host	Storage
	Shelf

Controller

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EK–SMCPQ–UG. C01 2–3