Cache

Enhanced Definition

Cache refers to a high-speed storage component that stores frequently accessed data, instructions, or control information to reduce latency and improve performance by minimizing access to slower main memory or disk storage. In the z/OS environment, caching is implemented at multiple levels, including CPU hardware, storage controllers, and various software components and applications.

Key Characteristics

- Hierarchical Structure: Modern IBM zSystems employ a multi-level CPU cache hierarchy (L1, L2, L3, L4, and sometimes L5) directly on or near the processor chips, managed by hardware to store instructions and data for rapid CPU access.
- Storage Controller Caching: DASD (Direct Access Storage Device) subsystems, such as the IBM DS8000 series, incorporate large amounts of non-volatile cache (NVC) to buffer I/O operations, significantly improving read and write performance for attached z/OS systems.
- Software Caching: z/OS components and applications (e.g., DB2 buffer pools, CICS data tables, VSAM buffers, IMS buffer pools) implement their own caches within main memory to reduce physical I/O to disk.
- Cache Coherency: Critical in multi-processor zSystems, hardware mechanisms ensure cache coherency, guaranteeing that all CPUs and LPARs (Logical Partitions) maintain a consistent view of shared data.
- Write-Back and Read-Ahead: Storage controllers often use write-back caching, acknowledging writes quickly and destaging data to disk later (protected by NVC), and read-ahead caching to prefetch data blocks anticipated to be requested soon.

Use Cases

- Database Performance Optimization: DB2 and IMS utilize extensive buffer pools (caches) in main memory to hold frequently accessed data pages and index blocks, drastically reducing physical I/O to DASD and improving query response times.
- Transaction Processing Acceleration: CICS applications frequently cache programs, maps, and data (e.g., CICS data tables, LRU pools) in memory to minimize I/O operations and enhance transaction throughput and response times.
- File System Efficiency: VSAM and zFS (z/OS UNIX File System) leverage buffers and caches to speed up file access, reduce I/O contention, and improve overall file system performance.
- Storage Subsystem Throughput: DASD controllers (e.g., IBM DS8000) cache frequently accessed data and buffer write operations, leading to significant improvements in overall I/O throughput and reduced latency for all attached z/OS systems.
- CPU Instruction and Data Access: Hardware caches (L1-L5) store instructions and data close to the CPU, minimizing the time spent accessing slower main memory and thereby boosting the instruction execution rate of z/OS workloads.

Related Concepts

Caching is fundamental to I/O performance on z/OS, directly impacting DASD (Direct Access Storage Device) response times by buffering reads and writes. It is closely tied to Virtual Storage management, as software caches reside in real storage which is mapped from virtual storage. Workload Manager (WLM) considers I/O activity, which is heavily influenced by caching, when making decisions about resource allocation and dispatching units of work. Furthermore, HiperDispatch and Processor Resource/System Manager (PR/SM) are designed to optimize CPU cache utilization across logical partitions (LPARs).

Best Practices:

Monitor Cache Hit Ratios: Regularly monitor DB2 buffer pool hit ratios, VSAM buffer usage, and storage controller cache statistics to identify bottlenecks and ensure optimal performance.
Size Caches Appropriately: Allocate sufficient memory for software caches (e.g., DB2 buffer pools, CICS data tables) based on workload analysis and performance monitoring to maximize cache hits and minimize physical I/O.
**Optimize Data