Cross Invalidate
In a z/OS sysplex, cross invalidate refers to the process where one system notifies other systems that a cached copy of a shared data block has become outdated or invalid due to an update made by the originating system. This mechanism is crucial for maintaining data integrity and cache coherency across multiple z/OS images accessing the same shared resources, such as shared DASD.
Key Characteristics
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- Sysplex-wide Coordination: It's a fundamental mechanism for ensuring data consistency across multiple z/OS images operating within a sysplex, especially when sharing data on common DASD volumes.
- Coupling Facility (CF) Dependent: Often relies on the z/OS Coupling Facility (CF) for broadcasting invalidation signals, particularly for shared data structures like those used by DB2 Data Sharing or IMS Data Sharing.
- Cache Coherency Protocol: Forms a critical part of the overall cache coherency protocol, guaranteeing that all systems in a sysplex always access the most current version of shared data.
- Performance Impact: While essential for integrity, frequent invalidations can introduce overhead, as systems must discard stale data from their local caches and potentially re-read it from disk or the CF.
- Automatic Process: Typically managed automatically by the z/OS operating system and specific subsystem components (e.g., DB2, IMS, VSAM RLS) rather than being explicitly coded by application programs.
- Granularity: Invalidations can occur at various levels of granularity, such as individual data pages, blocks, or records, depending on the specific data manager and its implementation.
Use Cases
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- DB2 Data Sharing: When a DB2 member updates a page in a shared DB2 group buffer pool (GBP), it sends an invalidate signal via the Coupling Facility to other members that might have that page cached, preventing them from using stale data.
- IMS Data Sharing: Similar to DB2, IMS data sharing environments utilize cross invalidation to maintain consistency of database blocks cached by different IMS systems accessing shared databases.
- VSAM Record Level Sharing (RLS): For VSAM datasets opened in RLS mode, z/OS employs cross invalidation to ensure that updates made by one system are immediately reflected in the caches of other systems accessing the same dataset.
- Shared HFS/zFS Filesystems: While not a direct "cross invalidate" in the traditional sense, shared file systems like zFS implement cache management and coherency protocols to ensure data consistency across sysplex members.
Related Concepts
Cross invalidation is a cornerstone of data sharing in a z/OS sysplex environment, directly enabling the functionality of Coupling Facility (CF) list and cache structures. It works in conjunction with cache coherency protocols to ensure that shared DASD resources, accessed by multiple z/OS images, maintain data integrity. This mechanism is critical for high-availability and scalability solutions like DB2 Data Sharing and IMS Data Sharing, preventing data corruption that could arise from stale cached data.
- Optimize CF Sizing and Connectivity: Ensure Coupling Facility structures are adequately sized and have robust, low-latency connectivity to minimize delays in invalidation signal propagation.
- Monitor Cache Hit Ratios: Regularly monitor buffer pool and cache hit ratios. High invalidation rates might indicate contention or inefficient data access patterns that could be optimized.
- Understand Subsystem Cache Management: Be aware of how specific subsystems (DB2, IMS, VSAM RLS) implement their cache coherency and invalidation mechanisms, as this knowledge is vital for performance tuning.
- Design for Data Locality: Where possible, design applications and data access patterns to promote data locality, thereby reducing the frequency and impact of cross invalidations across systems.
- Use Appropriate Sharing Levels: Choose the correct data sharing levels (e.g., VSAM RLS vs. non-RLS) based on application requirements and performance considerations, as different levels have varying invalidation overheads.