Adaptec 412R User Guide - Page 123

Theory of Controller Operation Disadvantages I Host HBA or the Fibre loop are single points of failure. I In single host configuration, the host system is also a single point of failure. I No upstream fail-over with a single host that has dual HBAs or dual host systems. I Protected LUN sharing between two host systems is not supported without the use of third-party file share/file lock software. Active-Passive Mode In the active-passive mode, there is one active and one passive RAID controller. The primary (active) controller operates with both of its FC host ports enabled and presents all LUNs on both ports to maximize bandwidth. The secondary (passive) controller keeps both of its FC host ports passive, and does not service any LUNs. Both controllers monitor each other's heartbeats and the passive controller receives write data and parity logs from the active controller. If the active controller fails, normal fail-over occurs to the passive controller, which becomes active by enabling both of its FC host ports and takes ownership of all arrays. Controller 1 is the default active controller. Controller 2 is always passive unless Controller 1 fails. The default active controller is not selectable. Because the active controller does not have to mirror the passive controllers data, more write-back cache memory can be made available to the active controller. This means that half the cache memory can be dedicated to writes and half to reads. This is the same allocation used in stand-alone mode. Each controller sends heartbeat messages via the SCSI buses to its partner controller. If a controller does not receive a heartbeat message within a set time period, it will kill the other controller assuming that it has malfunctioned. After killing the other controller, it will take ownership of the arrays and will activate its passive host port to assume the defunct controller's identity. A-21