A-21
Theory of Controller Operation
Disadvantages
■
Host HBA or the Fibre loop are single points of failure.
■
In single host configuration, the host system is also a single
point of failure.
■
No upstream fail-over with a single host that has dual HBAs or
dual host systems.
■
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.
DuraStor IUG - Online.book Page 21 Friday, July 19, 2002 3:06 PM
