Application planning
Before you start planning for an application, be sure you understand the data resources for your application and the location of these resources within the cluster in order to provide a solution that enables them to be handled correctly if a node fails.
To prevent a failure, you must thoroughly understand how the application behaves in a single-node and multinode environment. Do not make assumptions about the application's performance under adverse conditions.
Use nodes with the sufficient CPU cycles and I/O bandwidth to allow the production application to run at peak load. Remember, nodes should have enough capacity to allow PowerHA® SystemMirror® to operate.
To plan for this, benchmark or model your production application, and list the parameters of the heaviest expected loads. Then choose nodes for a PowerHA SystemMirror cluster that will not exceed 85% busy, when running your production application.
You can configure multiple application monitors for an application and direct PowerHA SystemMirror to both:
- Monitor the termination of a process or more subtle problems affecting an application
- Automatically attempt to restart the application and take appropriate action (notification or fallover) if restart attempts fail.
This section explains how to record all the key information about your application and begin drawing your cluster diagram.
- Lay out the application and its data so that only the data resides on shared external disks. This arrangement not only prevents software license violations, but it also simplifies failure recovery.
- If you are planning to include multitiered applications in parent-child dependent resource groups in your cluster, see the section Planning considerations for multitiered applications. If you are planning to use location dependencies to keep certain applications on the same node, or on different nodes, see the section Resource group dependencies.
- Write robust scripts to start and stop the application on the cluster nodes. The startup script especially must be able to recover the application from an abnormal end, such as a power failure. Ensure that it runs properly in a single-node environment before including the PowerHA SystemMirror software.
- Confirm application licensing requirements. Some vendors require a unique license for each processor that runs an application, which means that you must protect the license for the application by incorporating processor-specific information into the application when it is installed. As a result, even though the PowerHA SystemMirror software processes a node failure correctly, it might be unable to restart the application on the fallover node because of a restriction on the number of licenses for that application available within the cluster. To avoid this problem, be sure that you have a license for each system unit in the cluster that might potentially run an application.
- Ensure that the application runs successfully in a single-node environment. Debugging an application in a cluster is more difficult than debugging it on a single processor.
- Verify that the application uses a proprietary locking mechanism if you need concurrent access.