Replicated Data Cluster

In a replicated data cluster there is no shared disk. Instead, a data replication product synchronizes copies of data between nodes. Replication can take place at the application, host, and storage levels. Application-level replication products, such as Oracle DataGuard, maintain consistent copies of data between systems at the SQL or database levels. Host-based replication products, such as VERITAS Volume Replicator, maintain consistent storage at the logical volume level. Storage- or array-based replication maintains consistent copies of data at the disk or RAID LUN level.

Regardless of which replication technology is used, the solution must provide data access that is identical to the shared disks. If the failover management software requires failover due to a node or storage failure, the takeover node must possess an identical copy of data. This typically implies synchronous replication. At the same time, when the original server or storage is repaired, it must return to standby capability quickly to restore redundancy in the cluster. For example, if the replication solution must perform a full synchronization of data, redundancy may not be restored for an extended period.

The following illustration shows a hybrid shared storage/replicated data cluster, in which different failover priorities are assigned to nodes according to particular service groups.

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Shared Storage Replicated Data Cluster

Replicated data clusters can also be configured without the ability to fail over locally, but this configuration is not recommended. See Setting up a Replicated Data Cluster Configuration for more information.