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

Large volume support design considerations
Benefits of using large volumes can be briefly summarized as follows:
► They reduce storage management tasks by allowing you to define and manage smaller
configurations.
► They reduce the number of multivolume data sets you have to manage.
► They relieve architectural constraints by allowing you to address more data within the
existing 64K subchannel number limit.
The size of the logical volume defined does not have an impact on the performance of the
ESS subsystem. The ESS does not serialize I/O on the basis of logical devices, so an
increase in the logical volume size does not affect the ESS backend performance. Host
operating systems, on the other hand, serialize I/Os against devices. As more data sets
reside on a single volume, there will be greater I/O contention accessing the device. With
large volume support, it is more important than ever to try to minimize contention on the
logical device level. To avoid potential I/O bottlenecks on devices:
► Exploit the use of Parallel Access Volumes to reduce IOS queuing on the system level.
► Eliminate unnecessary reserves by using WLM in goal mode.
► Multiple allegiance will automatically reduce queuing on sharing systems.
Parallel Access Volume (PAV) support is of key importance when implementing large
volumes. PAV enables one MVS system to initiate multiple I/Os to a device concurrently. This
keeps IOSQ times down and performance up even with many active data sets on the same
volume. PAV is a practical “must” with large volumes. We discourage you from using large
volumes without PAV. In particular, we recommend the use of dynamic PAV and HyperPAV.
As the volume sizes grow larger, more data and data sets will reside on a single S/390 device
address. Thus, the larger the volume, the greater the multi-system performance impact will be
of serializing volumes with RESERVE processing. You need to exploit a GRS Star
configuration and convert all RESERVE's possible into system ENQ requests.
56 ABCs of z/OS System Programming Volume 3