Common Area

Enhanced Definition

In the context of IBM z/OS, a Common Area refers to a portion of virtual storage that is accessible and shared by multiple address spaces or programs. It serves as a repository for system-wide data, control blocks, and communication mechanisms, facilitating inter-program and inter-address space data exchange.

Key Characteristics

- Shared Access: Data stored in a common area can be read and potentially modified by multiple authorized programs running in different address spaces, promoting system-wide data consistency.
- Persistence: Data within common areas typically persists across the execution of multiple programs or even system IPLs (for certain types like SQA/ESQA), unlike private storage which is released when an address space terminates.
- Specific Storage Types: Common areas are implemented through specific virtual storage subpools, primarily the Common Storage Area (CSA), Extended Common Storage Area (ECSA), System Queue Area (SQA), and Extended System Queue Area (ESQA).
- Above/Below the Line: CSA and SQA reside below the 16MB line, while ECSA and ESQA reside above the 16MB line, allowing for larger storage allocations in 31-bit addressing mode.
- System-Managed: Allocation and management of common area storage are typically handled by the z/OS operating system through services like GETMAIN and FREEMAIN for specific subpools.
- Global Scope: Information stored in common areas often has a global scope, affecting the behavior or state of the entire system or a significant portion of it.

Use Cases

- System Control Blocks: Storing critical system control blocks (e.g., for I/O devices, task management, or resource managers) that need to be accessible to various system components.
- Inter-Address Space Communication: Facilitating communication and data exchange between different address spaces, such as passing parameters or status information between a system service and an application.
- Global Data Sharing: Maintaining global variables or configuration settings that need to be consistent and available to all or many programs across the system.
- Resource Management: Implementing system-wide resource management structures, like queues for shared resources or tables mapping system-wide identifiers.
- Performance Monitoring Data: Storing performance metrics or tracing information that can be collected and analyzed by system monitoring tools from various points in the system.

Related Concepts

Common areas are fundamental to the z/OS virtual storage architecture, working in conjunction with address spaces to enable multi-programming and shared resource management. They are distinct from private storage, which is exclusive to a single address space. The allocation and deallocation of common area storage are managed by the Virtual Storage Manager (VSM), often using services like GETMAIN and FREEMAIN. Specific common areas like CSA, ECSA, SQA, and ESQA are distinct regions within the common area, each with specific characteristics regarding protection and persistence, and are crucial for the operation of system services and subsystems like CICS and DB2.

Best Practices:

Minimize Usage: Allocate common area storage only when absolutely necessary, as excessive use can deplete critical system resources and impact overall system performance and stability.
Protect Data Integrity: Implement robust serialization mechanisms (e.g., ENQ/DEQ, latches, or program locks) to prevent data corruption due to concurrent updates from multiple programs.
Proper Allocation and Deallocation: Always free common area storage when it is no longer needed to prevent storage leaks, which can lead to system outages. Use appropriate GETMAIN and FREEMAIN calls.
Monitor Usage: Regularly monitor common area usage (e.g., using RMF, SMF, or system commands like D SMF,O) to detect potential shortages or abnormal growth that could indicate issues.