Evict

Enhanced Definition

In the context of IBM z/OS and mainframe systems, `evict` refers to the process of removing data, code, or control blocks from a specific storage area (such as a cache, buffer pool, or main memory) to free up space. This action is typically performed by the operating system or a subsystem when the storage area becomes full or resources are constrained, making room for new or more frequently accessed items.

Key Characteristics

- Resource Management: It is a fundamental mechanism for managing finite memory and storage resources efficiently within z/OS and its subsystems.
- Algorithm-Driven: Eviction decisions are often based on algorithms like Least Recently Used (LRU), Least Frequently Used (LFU), or other proprietary heuristics to determine which items to remove.
- Performance Impact: Frequent eviction of actively used items can lead to performance degradation, as those items must be reloaded from slower storage (e.g., disk) when needed again.
- System-Initiated: Eviction is typically an automatic process initiated by the z/OS Virtual Storage Manager (VSM) or by subsystems like DB2, IMS, or CICS.
- Not Deletion: Eviction does not permanently delete the data; it merely removes it from a faster, more volatile storage tier, implying the data still exists elsewhere (e.g., on disk).

Use Cases

- DB2 Buffer Pool Management: When a DB2 buffer pool reaches its capacity, older or less frequently accessed data pages are evicted to make space for new data pages being read from disk.
- CICS Dynamic Storage Areas (DSAs): CICS might evict less active transaction programs, maps, or temporary storage data from its DSAs to manage its memory footprint and prioritize active workloads.
- z/OS Paging: The Virtual Storage Manager (VSM) evicts (pages out) virtual storage pages from central storage to auxiliary storage (DASD) when real memory is scarce, making room for more active pages.
- Shared Memory Segment Management: In some scenarios, shared memory segments that are no longer actively used by any process might be evicted or swapped out by the system.

Related Concepts

Eviction is intrinsically linked to virtual storage management and memory management in z/OS, directly influencing paging activity. It is a critical component of how subsystems like DB2, IMS, and CICS manage their buffer pools, data spaces, and dynamic storage areas to optimize performance and handle resource constraints. Effective eviction policies minimize I/O operations by keeping frequently accessed data in faster memory, while inefficient eviction can lead to excessive I/O and degraded system responsiveness.

Best Practices:

Monitor Buffer Pool Hit Ratios: For DB2 and IMS, consistently monitor buffer pool hit ratios; low ratios often indicate excessive eviction and suggest a need to increase buffer pool sizes.
Tune Storage Allocations: Appropriately size buffer pools, DSAs, and other memory structures to minimize unnecessary eviction, balancing memory usage with performance requirements.
Analyze Paging Rates: High paging rates (especially page-outs) can be a symptom of frequent page eviction due to insufficient real memory, indicating a need for memory analysis or upgrade.
Optimize Application Locality: Design applications to exhibit good data and code locality, ensuring frequently accessed items remain in memory and reduce the likelihood of premature eviction.
Understand Subsystem Eviction Policies: Be aware of the specific eviction algorithms and parameters used by critical subsystems (e.g., DB2's LRU, CICS's storage management) to better diagnose and tune performance issues.

The term "Evolution - Gradual development" is a general concept and does not have a specific, technical definition or usage within the IBM mainframe, z/OS, COBOL, or JCL ecosystem that would allow for a dedicated glossary entry following the requested structure.

Glossary entries are typically for specific technical terms, components, processes, or software elements relevant to the domain. "Evolution" in a general sense describes the historical development of mainframe technologies (e.g., the evolution of z/OS from MVS, the evolution of COBOL standards, or the evolution of mainframe hardware from vacuum tubes to modern z-series processors), but it is not a technical term *itself* within the mainframe lexicon.

Therefore, I cannot generate a meaningful entry for "Evolution - Gradual development" that adheres to the strict mainframe-specific requirements for "Key Characteristics," "Use Cases," "Relationship to Other Concepts," and "Best Practices" as a standalone technical term.

Please provide a specific mainframe-related term for a glossary entry.