AUX - Auxiliary

Enhanced Definition

In the context of z/OS, "Auxiliary" primarily refers to **auxiliary storage**, which is the paging space on Direct Access Storage Devices (DASD) used by the z/OS Virtual Storage Manager. It serves as an extension of central storage (real memory), temporarily holding pages of virtual storage that are not currently active in real memory. This mechanism enables z/OS to support a virtual memory environment larger than the available physical memory.

Key Characteristics

- Paging Space: Its fundamental role is to provide storage for pages of virtual memory that have been "paged out" from central storage.
- DASD-based: Auxiliary storage resides on DASD volumes, typically defined as PAGE datasets, and is managed by the system.
- Virtual Storage Management: It is a critical component of z/OS's virtual storage management, allowing programs to address more memory than physically installed.
- Page-in/Page-out Operations: Data is moved between central storage and auxiliary storage via page-in (read) and page-out (write) operations.
- Performance Impact: The capacity, speed, and I/O characteristics of auxiliary storage directly influence overall system performance and responsiveness.
- System-Managed: The allocation and deallocation of pages within auxiliary storage are handled automatically by the z/OS Virtual Storage Manager.

Use Cases

- Virtual Memory Extension: Allows applications and the operating system to utilize a virtual address space much larger than the available real memory.
- Multiprogramming Support: Facilitates the concurrent execution of numerous address spaces (tasks, jobs, users) by providing an overflow area for inactive or less frequently accessed pages, freeing up real memory for active processes.
- Swapping Inactive Processes: Pages belonging to address spaces that are temporarily inactive or waiting for I/O are moved to auxiliary storage to optimize real memory usage.
- System Initialization (IPL): Paging datasets are defined and initialized during system IPL (Initial Program Load) to ensure the virtual storage environment is ready for operation.

Related Concepts

Auxiliary storage is intrinsically linked to central storage (real memory) and the Virtual Storage Manager (VSM). It forms the foundation of z/OS's virtual memory architecture, enabling multiprogramming and the efficient execution of multiple address spaces. High paging rates (the frequency of page-in/page-out operations) indicate heavy reliance on auxiliary storage, which can impact system performance if not adequately provisioned or optimized. It works in conjunction with Expanded Storage (if configured) as part of the memory hierarchy.

Best Practices:

Adequate Sizing: Ensure sufficient auxiliary storage capacity to prevent AUX storage shortages, which can lead to system abends (e.g., 878 or 80A) or severe performance degradation.
Performance Optimization: Distribute PAGE datasets across multiple DASD volumes, control units, and channels to minimize I/O contention and improve paging throughput.
Monitoring: Regularly monitor paging rates, auxiliary storage utilization, and page fault statistics using tools like RMF (Resource Measurement Facility) or SMF (System Management Facilities) to identify and address potential bottlenecks.
Dedicated Volumes: Consider placing PAGE datasets on dedicated or lightly used DASD volumes to isolate paging I/O from other application or system I/O, ensuring consistent performance.
High-Speed DASD: Utilize faster DASD technologies (e.g., SSDs if supported and practical for paging) for PAGE datasets to reduce page-in/page-out times and improve overall system responsiveness.