HVS - High Volume Sharing

Enhanced Definition

HVS, or High Volume Sharing, refers to a set of `VSAM` (Virtual Storage Access Method) sharing options and techniques designed to enable multiple `CICS` regions, `batch` jobs, or other applications to concurrently access and update the same `VSAM` `KSDS` (Key -Sequenced Data Set) or `RRDS` (Relative Record Data Set) with high transaction rates while maintaining data integrity and optimal performance. It is an enhancement over basic `VSAM` sharing for environments demanding high concurrency.

Key Characteristics

- High Concurrency Support: Specifically optimized for environments where numerous concurrent tasks or address spaces need to read from and write to the same VSAM file.
- Data Integrity Mechanisms: Employs sophisticated locking and serialization techniques (e.g., VSAM RLS or CICS RLS support, or VSAM share options with ENQ/DEQ mechanisms) to prevent data corruption during simultaneous updates.
- Performance Optimization: Aims to reduce I/O contention and improve throughput by efficiently managing buffer pools (LSR or GSR) and control intervals, minimizing physical I/O operations.
- Shared Buffering: Leverages LSR (Local Shared Resources) or GSR (Global Shared Resources) buffer pools to cache data and index blocks, allowing multiple users within an address space or across address spaces to share cached data.
- Specific VSAM Types: Primarily applicable to KSDS and RRDS datasets, which are commonly used for transactional data.
- Configuration Dependent: Requires specific VSAM DEFINE parameters (e.g., SHAREOPTIONS) and potentially CICS FILE resource definitions or IDCAMS statements to enable and optimize.

Use Cases

- Online Transaction Processing (OLTP): A CICS application processing thousands of customer inquiries, order entries, and updates against a shared VSAM customer master file or inventory file.
- Batch-Online Coexistence: Allowing a CICS region to update a VSAM file concurrently while a batch job reads from it for reporting or extracts, or even performs controlled updates.
- Inter-region Data Sharing: Multiple CICS regions within a sysplex accessing a common VSAM configuration file, lookup table, or shared work queue.
- High-Volume Logging/Auditing: A VSAM file used to log transactional events or audit trails from various applications or CICS systems.

Related Concepts

HVS is an advanced capability built upon VSAM (Virtual Storage Access Method), which is the fundamental file access method for indexed, relative, and entry -sequenced datasets on z/OS. It often works in conjunction with CICS (Customer Information Control System) to manage concurrent access to VSAM files, leveraging LSR (Local Shared Resources) or GSR (Global Shared Resources) buffer pools for efficient data caching. For even higher levels of sharing, especially across multiple z/OS systems in a sysplex, VSAM RLS (Record Level Sharing) is a more robust evolution that provides record-level locking and improved recovery mechanisms, often used in conjunction with SMS (Storage Management Subsystem) and DFSMS (Data Facility Storage Management Subsystem).

Best Practices:

Proper Buffer Pool Sizing: Carefully size LSR or GSR buffer pools (e.g., CICS DFHFCT parameters) to minimize I/O and optimize cache hit ratios, reducing contention.
Monitor Performance Metrics: Regularly monitor VSAM statistics (e.g., using SMF records, CICS statistics, RMF) to identify contention, buffer misses, I/O bottlenecks, and CI/CA splits.
Optimize Dataset Design: Design VSAM datasets with appropriate CI (Control Interval) and CA (Control Area) sizes, and allocate sufficient free space to reduce CI/CA splits and improve update performance.
Minimize Lock Duration: Design CICS transactions and batch