Workload management considerations for dynamic alias management

As part of the Enterprise Storage Subsystem's implementation of parallel access volumes, the concept of base addresses versus alias addresses is introduced. While the base address is the actual unit address of a given volume, there can by many alias addresses assigned to a base address, and any or all of those alias addresses can be reassigned to a different base address. With dynamic alias management, WLM can automatically perform those alias address reassignments to help work meet its goals and to minimize IOS queueing.

Note that to be able to move aliases to other bases, the aliases first have to have been initially assigned to bases via the ESS specialist, and the bases have to have come online at some point.

When you specify yes for this value on the Service Coefficient/Service Definition Options panel, you enable dynamic alias management globally throughout the sysplex. WLM will keep track of the devices used by different workloads and broadcast this information to other systems in the sysplex. If WLM determines that a workload is not meeting its goal due to IOS queue time, then WLM attempts to find alias devices that can be moved to help that workload achieve its goal. Even if all work is meeting its goals, WLM will attempt to move aliases to the busiest devices to minimize overall queueing.

In addition to automatically managing aliases, WLM ensures during system initialization that a minimum number of aliases is assigned to parallel access volumes with page data sets. The minimum number of aliases for a volume is:

2 * number_of_page_datasets - 1

This allows each page data set to have two I/Os active at a time, and ensures that paging intensive activities such as system dumping are not delayed by IOS queueing. This automatic enforcement of the minimum aliases only happens if dynamic parallel access volumes management is active for the device.

IMPORTANT: If you enable dynamic alias management, you must also enable I/O priority management. So you need to specify yes for both of these options on the panel. If I/O priority management is set to no, you will get only the efficiency part of dynamic alias management and not the goal-oriented management. This means that WLM will make alias moves that minimize overall IOS queueing, but these moves will not take service class goals into consideration.