SyncSort for z/OS and PAV
Posted by:Steve | Thu 30 October 2008
SyncSort for z/OS is a high-performance sort/merge/copy utility designed to exploit the advanced facilities of the z/OS operating system and zSeries computers. Three facilities recently introduced by IBM are Parallel Access Volume (PAV) technology, the Modified Indirect Address Word (MIDAW) facility and the System z9 Integrated Information Processor (zIIP). This series of articles examines the creative means by which SyncSort for z/OS exploits these facilities to reduce CPU time and elapsed time - first up is PAV.
What is PAV?
Parallel Access Volume technology was introduced to relieve the problem of UCB I/O serialization. Before the advent of PAV, jobs issuing I/O requests to the same DASD (mapped by a single UCB) could experience delays caused by the system limitation which allows only one active I/O request to transfer data between a DASD and the CPU.
Figure 1: System activity on a system without PAV active
PAV technology creates separate logical images of a DASD as alias UCBs. This technology directs I/O requests to alias UCBs instead of queuing the I/Os if the primary UCB is in use. Thus, multiple jobs can perform I/O concurrently to data sets residing on the same volume. This increases system throughput by decreasing I/O wait time. PAV technology has been described as the most important advance in I/O performance since caching.
Figure 2: System activity on a system with PAV active
How Does SyncSort for z/OS Exploit PAV?
While all jobs on a system can benefit from PAV technology, SyncSort for z/OS goes a step further. SyncSort developers created new I/O techniques to take advantage of the ability to issue multiple I/O requests to the same device simultaneously. These new techniques are used to automatically drive multiple concurrent I/Os to any SORTIN or SORTOUT data set which is allocated on a PAV-capable DASD volume. These concurrent I/O operations complete faster than would otherwise be possible using more conventional serial I/O. This parallel processing can reduce the elapsed time of a SyncSort application up to 30%.
Figure 3: SyncSort’s parallel I/O reduces elapsed time up to 30%
SyncSort handles the control and coordination necessary to manage these multiple concurrent I/Os. No user intervention is required. Users are typically not even aware that this processing is occurring. All they see is that their jobs with sorts are completing in a much shorter amount of time.
The original intent of PAV was to improve system throughput by reducing I/O queuing time. SyncSort’s innovative use of PAV improves elapsed time in a way that is unique among sorting products.
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