Storage Management

Virtual storage is the address space available to a program, providing each program with its own private address space. This creates the illusion that each program has access to a large, contiguous block of memory. Virtual storage is divided into regions and subpools, with the system managing the mapping between virtual addresses and real storage locations. This virtualization allows for better memory management, protection between programs, and the ability to run programs larger than available physical memory through paging.

Paging is a memory management technique that allows programs to use more memory than is physically available. When a program references a virtual address that is not in real storage, a page fault occurs, and the system brings the required page from auxiliary storage (DASD) into real storage. Conversely, when real storage becomes full, the system selects pages to move back to auxiliary storage. The paging process is transparent to applications and is managed by the Auxiliary Storage Manager (ASM). Effective paging requires careful tuning of page data sets and monitoring of page fault rates to ensure optimal performance.

DFSMS (Data Facility Storage Management Subsystem) is IBM's strategic storage management solution for z/OS, providing comprehensive storage management capabilities. It consists of several components:

• **DFSMSdfp**: Provides data set services and utilities
• **DFSMSdss**: Offers data set services for backup and recovery
• **DFSMShsm**: Manages hierarchical storage management
• **DFSMSrmm**: Handles removable media management

DFSMS uses Storage Management Subsystem (SMS) constructs like storage classes, management classes, and data classes to automate storage management decisions, reducing the need for manual intervention and ensuring consistent storage policies across the enterprise.

Data set encryption on z/OS provides transparent encryption and decryption of data at rest, protecting sensitive information stored on DASD. The encryption is performed by the I/O subsystem and is transparent to applications—no code changes are required. Encryption keys are managed through the Integrated Cryptographic Service Facility (ICSF), and the encryption process uses AES-256 encryption. Data set encryption can be applied at the data set level or through SMS policies, allowing organizations to encrypt specific data sets or all data sets in a storage class. The encryption/decryption is hardware-accelerated using CPACF for optimal performance.

The IBM Z storage hierarchy is designed to optimize performance and cost by using different storage technologies based on access patterns and data importance:

• **Real storage**: The fastest storage, consisting of main memory (RAM) directly accessible by processors
• **Expanded storage**: An optional extension of real storage, used as a high-speed paging device
• **DASD (Direct Access Storage Device)**: Primary auxiliary storage, including SSD and HDD technologies
• **Tape storage**: Cost-effective long-term storage for archival data

The Storage Management Subsystem (SMS) manages data movement between these levels automatically, ensuring frequently accessed data remains in faster storage while less active data migrates to slower, cheaper storage.

VSAM (Virtual Storage Access Method) is a high-performance data access method for z/OS that provides efficient storage and retrieval of data. VSAM data sets are organized in control intervals and control areas, providing:

• **KSDS (Key-Sequenced Data Set)**: Indexed data sets with direct access by key
• **ESDS (Entry-Sequenced Data Set)**: Sequential data sets with direct access by relative byte address
• **RRDS (Relative Record Data Set)**: Direct access by relative record number
• **LDS (Linear Data Set)**: Byte-addressable data sets for high-performance applications

VSAM provides better performance than traditional sequential access methods and supports features like data compression, encryption, and backup/recovery. It's particularly important for high-volume transaction processing systems like CICS and IMS applications.