Lotus Domino Administrator 8.5 Help - Choosing the server availability threshold

CLUSTERS

Choosing the server availability threshold
Setting the server availability threshold on each server is a key factor in balancing the workload in the cluster. Setting the server availability threshold too high can result in failover occurring unnecessarily. Setting it too low can result in poor performance for users who could have received better performance from a different server.

To determine the proper value for the server availability threshold, do the following:

1. During periods of normal to heavy load, use one of the following methods to observe the server availability index:

From the IBM® Lotus® Domino® Administrator or the Web Administrator, make the server you want current. Then click the Server - Statistics tab. Then, in the statistics list, expand Server.
In the Server pane of the Domino Administrator, expand All Servers or expand Clusters, right-click the server you want, choose Server Properties, and then click the Cluster tab.

This method is not available in the Web Administrator.

At the server console, type show cluster.
At the server console, type show stat server.

Note

Before using the next two methods, you must enable statistic reporting.

From the Domino Administrator or the Web Administrator, click the Server - Analysis tab. In the Task pane, expand Monitoring Results - Statistics Reports - Clusters. In the Results pane, open the Monitoring Results document you want.
Open the Monitoring Results database (STATREP.NSF), and look in the Statistics Reports - Cluster view.

2. Set an initial availability threshold based on the results of your observation. Consider the following when setting this value:

The value should be near the lower end of the values you observed.
Add some extra capacity (lower the availability threshold number) to accommodate servers that may fail over to this server.

When a server fails, the workload fails over to other servers in the cluster. If there are only two servers in the cluster, each with the same workload, this would result in approximately a 100% increase in the workload of the remaining server. If there are six servers in the cluster, this would result in approximately a 20% increase in the workload of the remaining servers. Therefore, you should set the availability threshold of each cluster server low enough to allow the server to absorb an adequate portion of the workload if another cluster server fails.

3. Track other cluster statistics to see if the workload is reasonably balanced. The following table lists some of the statistics that are helpful in determining if the workload is balanced.

Statistic name	Description
Server.AvailabilityIndex	The current value of the server availability index. The values range from 0 to 100. A value of 0 indicates that there are no resources available on the server. A value of 100 means that the server is completely available.
Server.ExpansionFactor	The current expansion factor. This value is used to compute the availability index. A value of 1 indicates that the server is completing transactions at the minimum time for that server. A value of 64 indicates that it is taking 64 times longer than the minimum time to complete transactions. By default, an expansion factor of 64 indicates a fully-loaded server, and results in an availability index of 0 (zero). You can use the NOTES.INI setting Server_Transinfo_Range to change the expansion factor value that indicates a fully loaded server. You can use the Show AI server command to obtain a suggested availability index.
Server.Cluster.OpenRedirects.LoadBalance.Successful	The number of times a BUSY server successfully redirects a client to another cluster member.
Server.Cluster.OpenRedirects.LoadBalance.Unsuccessful	The number of times a BUSY server is unsuccessful in redirecting a client to another cluster member. A server will be unsuccessful if the target server is also in a BUSY state or otherwise unavailable.
Server.Cluster.OpenRequest.ClusterBusy	The number of times a BUSY server tries to redirect a client request when all other cluster servers were also BUSY.
Server.Cluster.OpenRequest.LoadBalanced	The number of times a user tried to open a database on this server when the server was BUSY.

These statistics are cumulative from when the server was started.

4. Compare these statistics for all the servers in the cluster.

5. Adjust the server availability threshold on any servers that do not seem well balanced.

Note Workload balancing is not a substitute for having adequate computer resources for your enterprise. If your servers are already overloaded, workload balancing merely increases the problem because there is no place for a BUSY server to send client requests. Looking for an available server, however, decreases the performance on an already busy server.

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The server availability index

Setting the server availability threshold

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