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IBM-AUSTRIA - PC-HW-Support 30 Aug 1999 |
How a Cluster Works
How a Cluster Works
A cluster has two adapters, a primary and a non-primary adapter. The primary adapter
is the adapter that controls the RAID arrays.
When the cluster is first configured and the systems turned on, the adapter that has the higher SSA Unique ID is
automatically defined to be the primary adapter. The SSA configuration provides an option that allows the user to define the other
adapter to be the primary adapter, if required. All commands on the arrays, including commands from the non-primary adapter, are always
routed to the primary adapter for processing. The performance of I/O operations to RAID 1 arrays is:
- Highest when the server that contains the primary adapter starts all the I/O operatons.
- Lowest when the server that contains the non-primary adapter starts all the I/O operations.
All the arrays are reported to both systems. The system software determines which server communicates with which array.
Each adapter checks periodically that it can still communicate with its system. If it cannot communicate, it stops operating. The other
adapter detects that the first adapter has stopped operating, and reconfigures the SSA network.
If the non-primary adapter detects that is has lost access to the other adapter across the SSA loop, the non-primary adapter becomes the
primary adapter. Commands that were sent to the original primary adapter after access was lost are sent again, but this time, they are sent to the new
primary adapter. This action is called failover .
If write operations are in progress to an array (or have occurred within the previous 20 seconds) when failover occurs, the array
is rebuilt after the new primary adapter has taken control.
If an array ha one of its members missing (that is, the array is in the exposed or degraded state) when a failover occurs, the status of the array becomes
offline and an error is logged. Manual intervention is needed to resolve this error.
After a failover has occurred and a new adapter has been installed in place of the faulty one, the new adapter might have an SSA Unique ID that is
higher than that of the remaining (current primary) adapter. Under that condition, the new adapter becomes the primary adapter. This action is called failback .
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