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Guidelines for Managing Shadow Set Performance  



Sections Performance During Steady State and Performance During Copy and Merge Operations describe the performance impacts on a shadow set in steady state and while a copy or merge operation is in progress. In general, performance during steady state compares with that of a nonshadowed disk. Performance is affected when a copy or a merge operation is in progress to a shadow set. In the case of copy operations, you control when the operations are performed.

However, merge operations are not started because of user or program actions. They are started automatically when a system fails, or when a shadow set on a system with outstanding application write I/O enters mount verification and times out. In this case, the shadowing software reduces the utilization of system resources and the effects on user activity by throttling itself dynamically. Minimerge operations consume few resources and complete rapidly with little or no effect on user activity.

The actual resources that are utilized during a copy or merge operation depend on the access path to the member units of a shadow set, which in turn depends on the way the shadow set is configured. By far, the resources that are consumed most during both operations are the adapter and interconnect I/O bandwidths.

You can control resource utilization by setting the SHADOW_MAX_COPY system parameter to an appropriate value on a system based on the type of system and the adapters on the machine. SHADOW_MAX_COPY is a dynamic system parameter that controls the number of concurrent copy or merge threads that can be active on a single system. If the number of copy threads that start up on a particular system is more than the value of the SHADOW_MAX_COPY parameter on that system, only the number of threads specified by SHADOW_MAX_COPY will be allowed to proceed. The other copy threads are stalled until one of the active copy threads completes.

For example, assume that the SHADOW_MAX_COPY parameter is set to 3. If you mount four shadow sets that all need a copy operation, only three of the copy operations can proceed; the fourth copy operation must wait until one of the first three operations completes. Because copy operations use I/O bandwidth, this parameter provides a way to limit the number of concurrent copy operations and avoid saturating interconnects or adapters in the system. The value of SHADOW_MAX_COPY can range from 0 to 200. The default value is OpenVMS version specific.

Chapter 3 explains how to set the SHADOW_MAX_COPY parameter. Keep in mind that, once you arrive at a good value for the parameter on a node, you should also reflect this change by editing the MODPARAMS.DAT file so that when invoking AUTOGEN, the changed value takes effect.

In addition to setting the SHADOW_MAX_COPY parameter, the following list provides some general guidelines to control resource utilization and the effects on system performance when shadow sets are in transient states.


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