March 2013 Dell EqualLogic Configuration Guide v14.1 1-111.4.2 Controller redundancy in PS4100/PS6100 controllersEach array can be configured with either a single controller, or dual redundant controllers. The singlecontroller configuration will provide the same level of I/O performance as a dual controllerconfiguration. The dual controller configuration provides for redundancy. Redundant controllers willprevent volume connections between hosts and SAN from being dropped in the event of an activecontroller failure.TheActive Controller is the controller which is processing all disk and network I/O operations for thearray. A second controller in dual controller configurations will always be in a “passive” operatingmode. In this mode, the secondary controller will exhibit the following characteristics:• Each of its Ethernet ports are electrically inactive (active lights are off), unless a vertical portfailover has occurred.• The passive controller cache mirrors the cache of the active controller.1.4.3 Controller failover behavior: PS41x0/PS61x0In the event of a controller failure the following changes occur:• The passive controller immediately activates and continues to process all data requests to thearray.• Vertical port pairing insures that IP addresses assigned to each of the failed controller Ethernetports apply to the corresponding ports on the second controller.As stated in Section 1.3.3 above, controller models prior to the PS4100/PS6100 required having cabledconnections present on corresponding controller ports for controller failover to work correctly. Thevertical port failover feature in PS41x0/PS61x0 controllers eliminates this requirement, enablingpartially cabled configurations to support controller failover too. See Section 7.3.6,Minimum cablingscenarios: PS4100 and PS6100, for details.It is important to ensure that ports from each controller are connected to at least two differentswitches. This will prevent a switch failure from also disabling all paths between a host and itsconnected volumes. We illustrate controller failover behavior for the PS4100 family controller in Figure3. Controller failover behavior for the PS6100 (4 port) controller family is identical.IMPORTANT: To prevent a switch failure from also disabling all paths between a host andits connected volumes, you should make sure that ports from each controller areconnected to at least two different switches. You should also split vertical port pairconnections between two switches to ensure 100% bandwidth capability is maintainedin the event of a vertical port failover event. Both of these guidelines are illustrated inFigure 3.
