ISI - Inter-System Interconnect
Inter-System Interconnect (ISI) refers to the high-speed, direct communication links that facilitate data exchange and resource sharing between multiple z/OS systems, typically within a **Parallel Sysplex** or across different sysplexes. It provides the underlying physical connectivity crucial for shared data environments and workload distribution in enterprise mainframe computing.
Key Characteristics
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- High-Speed Connectivity: Designed for extremely low latency and high bandwidth to support synchronous operations and rapid data transfer between z/OS images.
- Hardware-Based: Implemented using dedicated hardware components such as Channel-to-Channel (CTC) adapters, Internal Coupling Facility (ICF) links, or specialized fiber optic connections.
- Sysplex Foundation: Forms the backbone of an IBM Parallel Sysplex, enabling the Coupling Facility (CF) to be accessed by all participating z/OS LPARs.
- Direct Communication: Provides direct paths between LPARs or between LPARs and the Coupling Facility, bypassing traditional network stacks for critical inter-system communication.
- Reliability and Redundancy: Often deployed with multiple redundant paths and robust error detection/correction mechanisms to ensure continuous availability.
- Scalability: Can be scaled by adding more links or higher-capacity adapters to accommodate increasing inter-system communication demands.
Use Cases
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- Parallel Sysplex Operation: Essential for the functioning of a Parallel Sysplex, allowing z/OS systems to share data, resources, and workloads through the Coupling Facility.
- Coupling Facility Connectivity: Links z/OS LPARs to the Coupling Facility (CF), enabling access to shared data structures, locking mechanisms, and messaging services critical for DB2 Data Sharing, IMS Data Sharing, and VSAM Record-Level Sharing (RLS).
- Cross-System Coupling Facility (XCF) Communication: Provides the physical transport for XCF messages and group services, facilitating inter-system coordination and communication for various z/OS components and applications.
- Geographically Dispersed Parallel Sysplex (GDPS): Long-distance ISI links (e.g., fiber optic) are used to connect sysplexes across different data centers for high availability and disaster recovery solutions.
- Direct LPAR-to-LPAR Communication: Can be used for specialized applications requiring direct, high-speed communication between specific z/OS LPARs without involving the Coupling Facility.
Related Concepts
ISI is fundamental to the Parallel Sysplex architecture, providing the physical layer for the Coupling Facility (CF) and Cross-System Coupling Facility (XCF). It enables z/OS LPARs to communicate with the CF for shared data access and resource serialization, which is critical for technologies like DB2 Data Sharing and IMS Data Sharing. ISI links, often implemented as CTC paths or ICF links, are the conduits through which XCF messages flow, allowing systems to coordinate and share status information. Without robust ISI, the high availability, scalability, and data integrity features of a Parallel Sysplex would not be possible.
- Implement Redundancy: Configure multiple, independent ISI paths (e.g., diverse CTCs or CF links) to eliminate single points of failure and ensure continuous operation in case of a link or adapter failure.
- Monitor Performance: Regularly monitor ISI link utilization, latency, and error rates using tools like RMF (Resource Measurement Facility) to identify bottlenecks and ensure optimal performance for critical sysplex operations.
- Capacity Planning: Perform thorough capacity planning for ISI bandwidth, especially for CF links, to accommodate peak workload demands and future growth, preventing performance degradation.
- Physical Security and Diversity: Ensure physical security of ISI cables and adapters, and route redundant paths through diverse physical conduits to protect against localized damage.
- Configuration Management: Meticulously document and manage ISI configurations, including port assignments, link speeds, and connection types, to facilitate troubleshooting and maintenance.