IBM N series Hardware Manual page 121

Failover because of disk mismatch
Communication between HA pair nodes is first established through the HA pair configuration
interconnect adapters. At this time, the nodes exchange a list of disk shelves that are visible
on the A loop and B loop of each node. If the B loop shelf count on its partner is greater than
its local A loop shelf count, the system concludes that it is impaired. It then prompts that
node's partner to start a takeover.
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Chapter 7. Highly Available controller pairs