DASD

Direct Access Storage Device

Enhanced Definition

Magnetic disk storage devices that allow direct access to any data location without sequential searching. DASD is the primary online storage medium for mainframe systems, providing random access to data.

Key Characteristics

- Direct access to any record without sequential reading
- Fixed and removable disk configurations
- Multiple concurrent access support
- High-speed data transfer rates
- Cached storage for performance optimization
DASD Types:
- 3390 Models: Current standard disk technology (3390-3, 3390-9, 3390-27, 3390-54)
- Enterprise Storage Systems (ESS): IBM's disk subsystems
- DS8000 Series: Current enterprise storage platforms
- Flash Storage: High-performance solid-state alternatives
- Virtual DASD: Storage virtualization implementations
Physical Organization:
- Volumes: Individual disk units or logical partitions
- Cylinders: Concentric tracks at same position
- Tracks: Circular data paths on disk surface
- Records: Individual data units on tracks
- Blocks: Physical units of data transfer
Capacity Characteristics:
- 3390-3: 2.8 GB per volume
- 3390-9: 8.5 GB per volume
- 3390-27: 27 GB per volume
- 3390-54: 54 GB per volume
Performance Factors:
- Seek time (arm movement)
- Rotational latency (disk rotation)
- Data transfer rate
- Cache hit ratio
- Channel bandwidth

Use Cases

- Online database storage (DB2, IMS)
- Active application datasets
- System software and libraries
- Temporary work files
- Catalog and directory information