HPA - High Performance Access

Enhanced Definition

High Performance Access (HPA) is a feature within IBM z/OS that significantly enhances the I/O performance of Virtual Storage Access Method (VSAM) datasets, particularly Key -Sequenced Data Sets (KSDSs) and Entry -Sequenced Data Sets (ESDS). It optimizes data access paths and reduces CPU overhead by leveraging in-memory caching for frequently accessed data and index blocks.

Key Characteristics

- VSAM Specific: Primarily designed to accelerate I/O operations for VSAM KSDSs and ESDS, which are fundamental data structures in z/OS.
- In-Memory Caching: Utilizes system memory (data spaces or hiperspaces) to cache data and index blocks, minimizing the need for physical I/O to Direct Access Storage Devices (DASD).
- CPU Reduction: By reducing physical I/O and optimizing access logic, HPA can significantly lower CPU consumption for applications accessing these files.
- Configuration Options: Can be enabled via JCL AMP parameters (e.g., ACCEL=I/O, ACCEL=R/W) for batch jobs or through File Control Table (FCT) entries in CICS.
- Application Transparency: Once configured, HPA operates transparently to the application program, which continues to use standard VSAM access methods.
- Read and Write Acceleration: Can accelerate both read-intensive and mixed read/write workloads, depending on the specific configuration chosen.

Use Cases

- High-Volume CICS Transactions: Accelerating access to critical VSAM files (e.g., customer master, product inventory) that are frequently read or updated by online CICS transactions to improve response times.
- Batch Processing Performance: Enhancing the performance of batch jobs that perform extensive sequential or random processing against large VSAM KSDSs or ESDS.
- Read-Intensive Reporting: Speeding up applications that generate reports by repeatedly reading data from VSAM files, reducing the elapsed time for report generation.
- Reducing I/O Contention: For shared VSAM files, HPA can reduce contention on DASD subsystems by serving requests from memory, especially beneficial in sysplex environments.

Related Concepts

HPA is an integral enhancement to VSAM, providing a performance boost to its core access methods, especially for KSDSs and ESDS. It is heavily utilized in CICS environments to improve online transaction response times by optimizing file I/O, and its activation for batch jobs is controlled via JCL AMP parameters. HPA leverages data spaces and hiperspaces to perform its in-memory caching, thereby reducing the number of physical I/O operations to DASD and improving overall system throughput.

Best Practices:

Selective Application: Apply HPA only to VSAM files that are genuinely I/O-bound and frequently accessed, as it consumes system resources (e.g., memory for caching).
Monitor Performance: Regularly monitor the effectiveness of HPA using SMF records and RMF reports to ensure it is delivering the expected performance benefits and not consuming excessive resources.
CICS FCT Configuration: For CICS, configure HPA in the File Control Table (FCT) for specific files using options like ACCELERATED(YES) rather than a blanket approach.
Appropriate JCL AMP Parameter: For batch, choose the AMP parameter carefully: AMP=('ACCEL=I/O') for read-intensive files and AMP=('ACCEL=R/W') for files with significant updates, understanding the implications for data integrity and consistency.
Buffer Allocation: Ensure sufficient VSAM buffer allocation (BUFND, BUFNI) as HPA works in conjunction with VSAM buffering to maximize performance gains.