first version of PAV the disk controller assigns a PAV to a UCB. In the second version of PAV processing, Workload Manager (WLM) reassigns a PAV to new UCBs from time to time. In the third version of PAV processing, with the IBM DS8000 series, each I/O uses any available PAV with the UCB it needs. The net effect of PAVs is to decrease the IOSQ time component of disk response time, often to zero. the only restrictions to PAV are the number of alias addresses, 255 per base address, and overall number of devices per logical control unit, 256 counting base plus aliases. Static versus dynamic PAVs There are two types of PAV alias addresses, static and dynamic. A static alias address is defined, in both DASD hardware and z/OS, to refer to a specific single base address. Dynamic means that the number of alias addresses assigned to a specific base address fluctuates based on need. The management of these dynamic aliases is left to WLM, running in goal mode (which is always the case with supported levels of z/OS). On most systems that implement PAV, there is usually a mixture of both PAV types. One, perhaps two, static aliases are defined for each base UCB and a bunch of dynamic aliases are defined for WLM to manage as it sees fit. As WLM watches over the I/O activity in the system, WLM determines if there a high-importance workload is delayed due to high contention for a specific PAV-enabled device. Specifically, for a disk device, base and alias UCBs must be insufficient to eliminate IOS Queue time. If there is high contention, and WLM estimates doing so would help the workload achieve its goals more readily, it will try to move aliases from another base address to this device. Another problem may be certain performance goals are not being met, as specified by WLM service classes. WLM will then look for alias UCBs that are processing work for less important tasks (service class), and if appropriate, WLM will re-associate aliases to the base addresses associated with the more important work. HyperPAVs WLM's actions in moving aliases from one disk device to another take a few seconds for the effects to be seen. For many situations this is not fast enough. HyperPAVs are significantly more responsive because they acquire a UCB from a pool for the duration of a single