What an HA pair is
An HA pair is two storage systems (nodes) whose controllers are connected to each other either directly or, in the case of a fabric-attached MetroCluster, through switches and FC-VI interconnect adapters. In this configuration, one node can take over its partner's storage to provide continued data service if the partner goes down.
You can configure the HA pair so that each node in the pair shares access to a common set of storage, subnets, and tape drives, or each node can own its own distinct set of storage.
The controllers are connected to each other through an HA interconnect. This allows one node to serve data that resides on the disks of its failed partner node. Each node continually monitors its partner, mirroring the data for each other’s nonvolatile memory (NVRAM or NVMEM). The interconnect is internal and requires no external cabling if both controllers are in the same chassis.
Takeover is the process in which a node takes over the storage of its partner. Giveback is the process in which that storage is returned to the partner. Both processes can be initiated manually or configured for automatic initiation.