ABCs of z/OS System Programming Volume 3 - IBM Redbooks

is the actual unit address of a given volume, there can be many alias addresses assigned to a
base address, and any or all of those alias addresses can be reassigned to a separate base
address. With dynamic alias management, WLM can automatically perform those alias
address reassignments to help work meet its goals and to minimize IOS queuing.
When you specify a yes 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 separate 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 queuing.
Alias assignment
It is not always easy to predict the volumes to have an alias address assigned, and how many.
Your software can automatically manage the aliases according to your goals. z/OS can exploit
automatic PAV tuning if you are using WLM in goal mode. z/OS recognizes the aliases that
are initially assigned to a base during the Nucleus Initialization Program (NIP) phase. WLM
can dynamically tune the assignment of alias addresses. WLM monitors the device
performance and is able to dynamically reassign alias addresses from one base to another if
predefined goals for a workload are not met. WLM instructs IOS to reassign an alias.
WLM goal mode management in a sysplex
WLM keeps track of the devices utilized by the various workloads, accumulates this
information over time, and broadcasts it to the other systems in the same sysplex. If WLM
determines that any workload is not meeting its goal due to IOSQ time, WLM attempts to find
an alias device that can be reallocated to help this workload achieve its goal.
Through WLM, there are two mechanisms to tune the alias assignment:
► The first mechanism is goal based. This logic attempts to give additional aliases to a PAV
device that is experiencing IOS queue delays and is impacting a service class period that
is missing its goal. To give additional aliases to the receiver device, a donor device must
be found with a less important service class period. A bitmap is maintained with each PAV
device that indicates the service classes using the device.
► The second mechanism is to move aliases to high-contention PAV devices from
low-contention PAV devices. High-contention devices will be identified by having a
significant amount of IOS queue. This tuning is based on efficiency rather than directly
helping a workload to meet its goal.
Chapter 3. Extended access volumes 61