Channel subsystem priority queuing

Channel subsystem priority queuing is an extension of the existing concept of I/O priority queuing. Previously, I/O requests were handled by the channel subsystem on a first-in, first-out basis. This could at times cause high priority work to be delayed behind low priority work. With Channel subsystem priority queuing, if important work is missing its goals due to I/O contention on channels shared with other work, it will be given a higher channel subsystem I/O priority than the less important work. This function goes hand in hand with the dynamic channel path management described — as additional channel paths are moved to control units to help an important workload meet goals, channel subsystem priority queuing ensures that the important workload receives the additional bandwidth before less important workloads that happen to be using the same channel.

WLM sets the priorities using the following basic scheme:

System related work is given the highest priority
High importance work missing goals is given a higher priority than other work
Work meeting goals is managed so that light I/O users will have a higher priority than heavy I/O users
Discretionary work is given the lowest priority in the system.

Channel subsystem priority queuing requires z/OS® and a System z® server in z/Architecture® mode. It works in both basic mode and LPAR mode, and does not require a coupling facility structure.