The nonvolatile memory (NVRAM or NVMEM, depending on the platform model) in the storage controllers is mirrored both locally to a local HA partner and remotely to a remote disaster recovery (DR) partner on the partner site. In the event of local failover or switchover, this configuration ensures that data in the nonvolatile cache is preserved.
In an HA pair that is not part of a clustered MetroCluster configuration, each controller maintains two caches: one for itself and one for its HA partner.
In a four-node MetroCluster configuration, the nonvolatile cache of each storage controller is divided into four partitions. In a two-node MetroCluster configuration, the HA partner partition and DR auxiliary partition are not used, because the controllers are not configured as an HA pair.
| Nonvolatile caches for a storage controller | |
|---|---|
| In a MetroCluster configuration | In a non-MetroCluster HA pair |
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In a two-node MetroCluster configuration, there is no HA partner partition because the controllers are not configured as an HA pair.
The DR partner is a node in the partner cluster that is paired with the local node.
The DR auxiliary partner is the HA partner of the local node's DR partner. This cache is needed if there is an HA takeover (either when the configuration is in normal operation or after a MetroCluster switchover).
In a two-node MetroCluster configuration, there is no DR auxiliary partner partition because the controllers are not configured as an HA pair.
For example, the local cache of a node (node_A_1) is mirrored both locally and remotely at the MetroCluster sites. The following illustration shows that the local cache of node_A_1 is mirrored to the HA partner (node_A_1) and DR partner (node_B_1):
If a local HA takeover occurs in a four-node MetroCluster, the taken-over node can no longer act as a mirror for its DR partner. To allow DR mirroring to continue, the mirroring automatically switches to the DR auxiliary partner. After a successful giveback, mirroring automatically returns to the DR partner.
For example, node_B_1 fails and is taken over by node_B_2. The local cache of node_A_1 can no longer be mirrored to node_B_1. The mirroring switches to the DR auxiliary partner, node_B_2.
