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NVRAM and NVMEM cache mirroring in a MetroCluster configuration

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

The caches store the following content:

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):

Dynamic mirroring in event of a local HA takeover

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.