The controller has two Single Inline Memory Module (SIMM) connectors for up to 64 MB of cache memory. The SIMM connectors form a mirrored pair when the controllers are configured in a redundant controller configuration; otherwise they are fully accessible by the controller. In a redundant controller setup, both controllers must have identical cache configurations and the total usable cache (per controller) will be half the amount installed. Thus, in a single controller setup, the maximum usable cache is 64 MB while a redundant setup has a maximum usable cache of 32 MB (per 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 passive 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 (that is 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. Redundancy 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.