HDA - Hard Disk Assembly

Enhanced Definition

In the context of IBM mainframe systems, an HDA (Hard Disk Assembly) refers to the sealed, electromechanical unit that forms the core physical component of a Direct Access Storage Device (DASD). It contains the magnetic platters, read/write heads, and the motor mechanism responsible for storing and retrieving data persistently for the z/OS operating system and its applications.

Key Characteristics

- Physical Storage Unit: The fundamental physical component responsible for magnetic data storage within a DASD unit.
- Sealed Design: Historically, HDAs were sealed units to protect internal components from contamination, enhancing reliability and preventing field servicing of internal parts.
- Platters and Heads: Comprises multiple magnetic platters mounted on a spindle and corresponding read/write heads that move across the platter surfaces.
- Part of a Larger Subsystem: An HDA is a component *within* a larger DASD subsystem (e.g., IBM 3380, 3390, or modern virtualized DASD arrays that abstract physical drives).
- Data Medium: Provides the physical medium where data is recorded in a format compatible with mainframe architectures, typically Count Key Data (CKD).
- Capacity Contributor: The number and density of platters within an HDA determine a portion of the overall storage capacity of a DASD volume.

Use Cases

- Primary Data Storage: Serving as the underlying physical storage for critical z/OS datasets, including sequential files, VSAM files, PDS/PDSE libraries, and system files.
- Database Storage: Housing tablespaces and index spaces for mainframe databases like DB2 for z/OS and IMS DB.
- System Volumes: Providing the physical foundation for z