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

not prepared to allow multiples I/Os due to software product compatibility issues. Support is
then implemented by defining multiple UCBs for the same device.
The UCBs are of two types:
► Base address: This is the actual unit address. There is only one for any volume.
► Alias address: Alias addresses are mapped back to a base device address. I/O scheduled
for an alias is executed against the base by the controller. No physical disk space is
associated with an alias address. Alias UCBs are stored above the 16 MB line.
PAV benefits
Workloads that are most likely to benefit from PAV functionality being available include:
► Volumes with many concurrently open data sets, such as volumes in a work pool
► Volumes that have a high read to write ratio per extent
► Volumes reporting high IOSQ times
Candidate data sets types:
► Have high read to write ratio
► Have many extents on one volume
► Are concurrently shared by many readers
► Are accessed using media manager or VSAM-extended format (32-byte suffix)
PAVs can be assigned to base UCBs either:
► Manually (static) by the installation.
► Dynamically, WLM can move aliases’ UCBs from one base UCB to another base UCB in
order to:
– Balance device utilizations
– Honor the goal of transactions suffering I/O delays because long IOSQ time. All WLMs
in a sysplex must agree with the movement of aliases’ UCBs.
However, the dynamic PAV still has issues:
► Any change must be decided by all WLMs in a sysplex using XCF communication.
► The number of aliases for one device must be equal in all z/OS systems.
► Any change implies a dynamic I/O configuration.
To resolve such issues HyperPAV was introduced. With HyperPAV all aliases’ UCBs are
located in a pool and are used dynamically by IOS.
Chapter 3. Extended access volumes 63