Cross Partition - Spanning LPARs

Enhanced Definition

In the context of IBM mainframes, "cross-partition" or "spanning LPARs" refers to processes, resources, or communication mechanisms that operate, interact, or extend across multiple Logical Partitions (LPARs) running on one or more physical Central Processor Complexes (CPCs). This design allows for shared resources, distributed workloads, and enhanced availability beyond the scope of a single LPAR.

Key Characteristics

- Inter-LPAR Communication: Requires specialized communication protocols and hardware, such as the Cross-System Coupling Facility (XCF) or Channel-to-Channel (CTC) adapters, to enable data exchange and synchronization between LPARs.
- Shared Resource Access: Often involves multiple LPARs concurrently accessing shared resources like DASD volumes, Coupling Facility (CF) structures, or network interfaces.
- System Complexity: Introduces increased complexity in design, configuration, management, and problem determination compared to single-LPAR environments.
- Performance Considerations: Communication across LPARs can incur overhead, making efficient design and minimization of cross-LPAR traffic crucial for optimal performance.
- Enhanced Availability/Scalability: Facilitates high availability by allowing workloads to fail over or be distributed across multiple LPARs, and enables horizontal scaling of applications.

Use Cases

- DB2 Data Sharing: Multiple DB2 subsystems in different LPARs accessing and updating the same set of DB2 data, coordinated by the Coupling Facility for data integrity and locking.
- IMS Data Sharing: Similar to DB2, allowing multiple IMS subsystems across LPARs to share and update the same IMS databases.
- Workload Balancing: Using a Sysplex Distributor or other workload managers to distribute incoming network connections or transactions across multiple application instances running in different LPARs.
- High Availability: Running redundant instances of critical applications or services in separate LPARs, allowing for automatic failover in case of an LPAR outage.
- Shared Storage Access: Multiple LPARs accessing the same DASD volumes, typically managed by Global Resource Serialization (GRS) or MIM for data integrity.

Related Concepts

The concept of "cross-partition" is fundamental to an IBM Parallel Sysplex environment, where multiple LPARs cooperate as a single system image. It heavily relies on the Cross-System Coupling Facility (XCF) for inter-LPAR communication and the Coupling Facility (CF) for shared data, locking, and messaging services. Shared DASD is a common component, requiring robust serialization mechanisms like GRS to prevent data corruption when multiple LPARs access the same volumes. Technologies like DB2 Data Sharing and IMS Data Sharing are prime examples of applications designed from the ground up to operate in a cross-partition manner.

Best Practices:

Minimize Cross-LPAR Communication: Design applications and system configurations to reduce the frequency and volume of data exchanged between LPARs to optimize performance.
Leverage Sysplex Services: Utilize XCF and Coupling Facility services for robust, high-performance, and resilient inter-LPAR communication and resource sharing.
Implement Robust Serialization: Ensure proper data integrity by using Global Resource Serialization (GRS), ENQ/DEQ mechanisms, or application-specific locking (e.g., DB2/IMS locking) when multiple LPARs access shared resources.
Comprehensive Monitoring: Implement extensive monitoring for XCF paths, Coupling Facility structures, shared resource contention, and application performance across all participating LPARs.
Security Configuration: Apply consistent and robust security policies across all LPARs involved in cross-partition operations to protect shared resources and communication paths.