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Auxiliary Storage

Enhanced Definition

Auxiliary storage, also known as secondary storage, refers to storage devices external to the mainframe's main memory (central storage or real storage). Its primary purpose in z/OS is to provide persistent, high-capacity storage for programs, data, and system files, overcoming the volatility and size limitations of main memory.

Key Characteristics

- Persistence: Data stored on auxiliary devices remains intact even after the system is powered off, unlike volatile main memory.
- High Capacity: Offers significantly larger storage capacities compared to main memory, supporting vast amounts of enterprise data and applications.
- Slower Access: Access speeds are considerably slower than main memory, requiring I/O operations to transfer data between auxiliary storage and main memory.
- Cost-Effective: Generally much less expensive per byte than main memory, making it economical for storing large volumes of data.
- Device Types: Primarily implemented using Direct Access Storage Devices (DASD) like disk drives (e.g., IBM DS8000 series) and magnetic tape drives.
- Block-Oriented: Data is typically read from and written to auxiliary storage in fixed-size blocks or records, managed by the operating system's I/O supervisor.

Use Cases

- Application Data Storage: Storing critical business data for applications, including flat files, VSAM datasets, and databases like DB2, IMS, and sequential files.
- Program Libraries: Housing executable programs (load modules), source code libraries (PDS/PDSE), JCL libraries, and system utilities.
- Virtual Storage Paging: z/OS utilizes auxiliary storage (specifically PAGE datasets on DASD) to manage virtual storage, moving inactive pages of memory between real storage and auxiliary storage.
- Backup and Recovery: Magnetic tapes and DASD are extensively used for creating backups of critical system data, applications, and databases for disaster recovery and archival purposes.
- Spooling Operations: JES (Job Entry Subsystem) uses auxiliary storage to spool job input (SYSIN), intermediate job output, and final job output (SYSOUT) before printing or display.

Related Concepts

Auxiliary storage works in conjunction with Main Memory (Real Storage), providing the persistent layer where data resides when not actively being processed by the CPU. The Virtual Storage mechanism in z/OS relies heavily on auxiliary storage, using PAGE datasets to swap memory pages in and out of real storage. Data transfer between auxiliary storage and main memory is managed by the I/O Subsystem, involving channels, control units, and device drivers. Data Management Services (like VSAM, QSAM, BSAM) provide the programming interfaces for applications to read and write data to various auxiliary storage devices.

Best Practices:

Performance Optimization: Optimize dataset placement on DASD to minimize I/O contention, use appropriate block sizes, and distribute I/O workload across multiple devices and channels.
Capacity Planning: Regularly monitor auxiliary storage usage and plan for future growth to prevent out-of-space conditions, which can halt critical applications or the entire system.
Backup and Recovery Strategy: Implement a robust backup and recovery strategy utilizing auxiliary storage (e.g., tape backups, DASD mirroring, replication) to ensure data availability and business continuity.
Data Security: Protect data on auxiliary storage through access control mechanisms (e.g., RACF), data encryption (both at rest and in flight), and physical security measures for storage devices.
Virtual Storage Management: Properly size and configure PAGE datasets on high-performance DASD to ensure efficient virtual storage operations and optimal system performance.