3.4. RAC/GFS Considerations

Oracle Clusterware implements Virtual IP routing so that target IP addresses of the failed node can be quickly taken over by the surviving node. This means new connections see little or no delay.

In the GFS/RAC cluster, Oracle uses the back-side network to implement Oracle Global Cache Fusion (GCS) and database blocks can be moved between nodes over this link. This can place extra load on this link, and for certain workloads, a second dedicated backside network might be required.

Bonded links using LACP (Link Aggregation Control Protocol) for higher capacity, GCS links, using multiple GbE links are supported, but not extensively tested. Customers may also run the simple, two-NIC bond in load-balance, but the recommendation is to use this for failover, especially in the two-node case.

Oracle GCS can also be implemented over Infiniband using the Reliable Data Sockets (RDS) protocol. This provides an extremely low latency, memory-to-memory connection. This strategy is more often required in high node-count clusters, which implement data warehouses. In these larger clusters, the inter-node traffic (and GCS coherency protocol) easily exhausts the capacity of conventional GbE/udp links.

Oracle RAC has other strategies to preserve existing sessions and transactions from the failed node (Oracle Transparent Session and Application Migration/Failover). Most customers do not implement these features. However, they are available, and near non-stop failover is possible with RAC. These features are not available in the Cold Failover configuration, so the client tier must be configured accordingly.

Oracle RAC is quite expensive, but can provide that last 5% of uptime that might make the extra cost worth every nickel. A simple two-node Red Hat Cluster Suite Oracle Failover cluster only requires one Enterprise Edition license. The two-node RAC/GFS cluster requires two Enterprise Edition licenses and a separately priced license for RAC (and partitioning).