RAID Array 3000 Pedestal Enclosure
2–20
EK–SMCPO–UG. A01
2.8
Redundant Operation
When operating in a redundant configuration, the two controllers are linked such
that, in case of a failure, the surviving controller can access the other controller’s
cache memory and complete all operations that were in progress when the failure
occurred. The controllers support two different configurations:
•
ACTIVE / ACTIVE: One host port is active on each controller. The other
port on each controller is passive and only used if the peer controller fails.
•
ACTIVE / PASSIVE: Both host ports on one controller are active. The
other controller’s ports are both passive and only used if the primary control-
ler fails.
When one controller fails, the survivor will process all I/O requests until the
failed controller is repaired and powered on. The subsystem will then return to its
previous state (i.e., ACTIVE / ACTIVE or ACTIVE / PASSIVE).
2.8.1 Initialization
During initialization, the firmware in the RAID 3000 verifies that both control-
lers have consistent configurations including identical memory cache and system
parameters. If the controller setups are incompatible, the set is not bound and
each controller operates in stand-alone mode.
2.8.2 Message Passing
Information is shared between the two controllers by a collection of messages
passed through the backplane connectors. The messages provide configuration
data as well as a heartbeat which is transmitted by each controller every 500 ms.
If a controller does not receive a heartbeat within one second, it assumes the peer
controller has become inoperable and begins failing over.
If the controllers cannot exchange messages due to communication problems
over the backplane, they will break the connection and each controller will
switch to a stand-alone mode.
2.8.3 Failover
Failover describes the process of transferring data from a failed controller to a
survivor and completing any active tasks. When one controller begins the fail-
over process, it sends a reset to the other controller, which prevents the failing
unit from processing any more information and enables any host ports that are
passive. It then downloads the failed controller’s cache to its unused portion of
cache and begins acting upon that data.
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