If your organization is small, you can start with two servers and add servers as your enterprise grows without affecting the performance to your users. Keep in mind that each server you add creates additional network traffic when it probes other cluster servers to find out their status and when it does cluster replication. Therefore, do not add servers to a cluster until you need the additional capacity or additional redundancy.

In a larger organization, you must decide whether to create large clusters or small clusters. A larger cluster is better able to absorb the workload when a cluster server fails. If you have a cluster with only two servers, for example, if one of the servers fails, the other server must absorb 100% of the failed server's workload. That means that you could run each server at only 50% of its capacity so that it has enough capacity available to absorb the workload of the other server. If the cluster has six servers, however, each of the remaining five servers must absorb only 20% of the failed server's workload. That means you could run each server at 80% of capacity, and they would still be able to absorb the workload if a server goes down. (Of course, there are other factors that determine how the workload of a failed server is absorbed, such as the way you have distributed replicas across the cluster servers.)

Hardware considerations

The number of servers you decide to include in a cluster can be affected by the amount of disk space and the processing power of each server. Keep the following in mind as you decide which hardware to use in your cluster:

• The more replicas you create, the more disk space you need and the more processing power you need for cluster replication.
• The Cluster Database Directory requires approximately 2MB of disk space plus an additional 1MB for each 2,000 databases in the cluster.
• The more servers in the cluster, the more processing power each server uses to communicate with the other cluster servers.
• The more server tasks and CPU-intensive applications you run on a server, the more processing power you need.
• Each server needs adequate processing power for the databases it contains and for any databases that might fail over to the server.
• Clustered servers require more memory than nonclustered servers. The actual amount you will require depends on the level of activity on the server.

  To see if you need additional memory or processing power on your computer, check the Platform statistics.

• When you have a large cluster or a cluster with a heavy workload, you might need to use multiple Cluster Replicators to improve the performance of cluster replication. Check the Cluster Replicator statistics to see if there is a large queue of databases waiting to be processed. If so, add Cluster Replicators one at a time until the statistics improve adequately.

  Because Cluster Replicators use system resources, the overall performance of the system could decrease as you add Cluster Replicators. Therefore, do not add more Cluster Replicators than you need.

Glossary