Data replication is the copying of data from a primary cluster to a backup or secondary cluster. Through data replication, the secondary cluster has an up-to-date copy of the data on the primary cluster. The secondary cluster can be located far away from the primary cluster.
Failover is the automatic relocation of a resource group or device group from a primary cluster to a secondary cluster. If the primary cluster fails, the application and the data are immediately available on the secondary cluster.
This section describes the remote mirror replication method and the point-in-time snapshot method used by Sun StorEdge Availability Suite software. This software uses the sndradm(1RPC) and iiadm(1II) commands to replicate data. For more information about these commands, see the Sun Cluster 3.0 and Sun StorEdge Software Integration Guide.
Remote mirror replication is illustrated in Figure 6–1. Data from the master volume of the primary disk is replicated to the master volume of the secondary disk through a TCP/IP connection. A remote mirror bitmap tracks differences between the master volume on the primary disk and the master volume on the secondary disk.
Remote mirror replication can be performed synchronously in real time, or asynchronously. Each volume set in each cluster can be configured individually, for synchronous replication or asynchronous replication.
In asynchronous data replication, a write operation is confirmed as complete before the remote volume is updated. Asynchronous data replication provides greater flexibility over long distances and low bandwidth.
Point-in-time snapshot is illustrated in Figure 6–2. Data from the master volume of each disk is copied to the shadow volume on the same disk. The point-in-time bitmap tracks differences between the master volume and the shadow volume. When data is copied to the shadow volume, the point-in-time bitmap is reset.
The following figure illustrates how remote mirror replication and point-in-time snapshot are used in Example Configuration.