Maximizing performance throughput is just as important today for organizations using the z/VSE operating system as it was in the past. A well-tuned z/VSE system permits maximum workloads to be processed in the shortest time possible, thereby permitting organizations to reap the full benefit of their mainframe hardware and software investment. One area that is always important to monitor and tune is storage I/O performance.

It is not unusual for storage devices to be a bottleneck for overall throughput of a z/VSE system. Since most data being accessed on a day-to-day basis is stored on disk devices, and multiple applications may need simultaneous access to the same data stored on those devices, I/O operations to storage devices can be significantly hampered as each application waits its turn for access to the data it needs. The good news is that the use of the Parallel Access Volume (PAV) feature in z/VSE can minimize or eliminate this problem.

Parallel Access Volume Feature

The PAV feature enables your system to access volumes in parallel when using an IBM System z host. PAV is an optional licensed feature of the System Storage DS8000 series.

PAV enables the storage controller to support concurrent data transfer operations from, or to, the same system. This can result in a significant performance improvement due to reduced device queue delays to busy devices. You must configure both the storage unit and the operating system to use PAVs. You can use the logical configuration definition to define PAV-bases, PAV-aliases and their relationship in the storage unit hardware. This unit address relationship creates a single logical volume, allowing concurrent I/O operations.

PAV Support for z/VSE

Initially PAV support for ECKD devices was introduced with z/VSE 4.2. There is no PAV support for FBA/SCSI devices. In z/VSE, a PAV volume has a volume serial number and 1-8 subchannels. This is one subchannel for the PAV-base and up to seven subchannels for the PAV-aliases. The relationship between PAV-base and PAV-alias addresses remains unchanged (static PAV). There is one device queue per subchannel, which enables the system to run multiple I/O requests simultaneously. The z/VSE supervisor will balance I/O traffic from different tasks upon the available subchannels by an internal load balancing algorithm.

In z/VSE, the PAV support can be turned on and off by the SYSDEF SYSTEM, PAV=START/ STOP command. The support is enabled by PAV=START. Default is PAV=STOP. The support is transparent to application programs.

Enabling PAV Support

Before you can use PAV support in z/VSE, you need to purchase, install and enable the PAV feature on the IBM System Storage Disk system. Furthermore, the PAV-base and PAV-aliases need to be defined in the Input/Output Configuration Data Set (IOCDS). A PAV-base must be defined in the z/VSE IPL start-up procedure (ADD statement) whereas PAV-aliases cannot be added or otherwise used in the z/VSE system. An ADD statement for a PAV-alias will be ignored by the system. After IPL, the operator has to enable the PAV support by the SYSDEF SYSTEM,PAV=START command.

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