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IBM DS8880 zHyperLink Technology Provides I/O Enhancements on the IBM z14

IBM DS8880 zHyperLink

VSAM applications that need to be restarted after an application or system failure typically invokes VSAM VERIFY processing before the application can resume. Lower I/O latency allows VSAM VERIFY processing to complete more quickly, thus improving availability without any application changes. Figure 1 shows the result of an experiment that demonstrates a two-thirds reduction in elapsed time for VSAM VERIFY processing.

I/O Latency Improvement Technologies

Prior to the IBM Z family of processors, the IBM z14 and IBM DS8880 Storage System with zHyperLink technology, I/O latency was primarily thought about only as the I/O response. This would include software queueing delays for an I/O request and the components of I/O service time. IBM Z FICON I/O technology provides the ability to parse the components of I/O service time. This ability simplifies the chore for clients to manage their mainframes systems by providing the ability to pinpoint potential problem areas, diagnose problems, and provide a mechanism for chargeback and accounting.

The components of I/O response time fall into four major areas: IOS queue time, pending time, connect time and disconnect time. However, much of the total time for the application to get results from the I/O operation has never been included as part of I/O service time. This additional time includes the time for PR/SM to dispatch the LPAR after the I/O interrupt occurs, the time for z/OS to dispatch the application that made the I/O request and the time it takes for the processor L1/L2 cache to get repopulated for the application reference set after other work that ran on the CPU polluted the processor cache.

The IBM DS8000 Storage System, z/OS, IBM Z processors and middleware (e.g., Db2 for z/OS) has implemented many technologies over the last few years to deliver a cadence of incremental improvements for I/O latency (see Figure 2). These include techniques to improve the cache hit ratios in the DS8000 Storage Systems, comprehensive workload management algorithm with the DS8000 I/O Priority Manager to provide policy-based goal management for workloads, improved I/O parallelism with HyperPAV and IBM zHyperWrite, eliminating unnecessary synchronization points in the I/O execution, faster I/O transport speeds with improved reliability and I/O protocol enhancements for improved efficiencies.

However, the heritage z/OS and z/Architecture I/O model has additional delays that contribute to the latency as seen by applications requesting I/O that hadn’t been directly addressed, including:

  • I/O interrupt delay time: When waiting for an I/O operation to complete, it’s possible for the LPAR to stop executing. The time from when the I/O interrupt occurs to the point in time when z/OS starts running and issues the instruction to retrieve the status is captured and reported by RMF as I/O interrupt delay time. I/O Interrupt delay time is a measure of the virtualization overhead in the system. This time maybe large because of the amount of contention for CPU resources.
  • z/OS dispatcher queueing time: After an I/O interrupt is processed by z/OS, the waiting middleware or application needs to be dispatched to process the results and continue execution. Clients can typically run their z/OS systems at very high CPU utilization.
  • Processor L1/L2 cache reload time: The heritage asynchronous I/O model allows other applications and units of work to run on the processor causing pollution of the processor L1/L2 cache. When the application resumes processing after the I/O request completes application incurs delays while the CPU reloads the cache.

Matt Kalos is an IBM Distinguished Engineer for storage development.

Harry M. Yudenfriend is an IBM Fellow with Systems and Technology Group, System z and Power who joined IBM in 1980. He was named an IBM Master Inventor in 2001 and has achieved his 33rd invention plateau.

Peter Sutton is an IBM Distinguished Engineer for IBM Z core technology.

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