Geometry - Spatial Layout

DASD Geometry

Enhanced Definition

In the context of IBM mainframe systems, "Geometry" primarily refers to the physical and logical organization of data on Direct Access Storage Devices (DASD), such as disk drives. It defines the spatial layout of storage units like cylinders, tracks, and blocks/records, which dictates how data is physically addressed and accessed. This geometry is fundamental to storage allocation, I/O performance, and data management on z/OS.

Key Characteristics

- Cylinder: A vertical stack of tracks located at the same radial distance from the spindle across all platters of a disk pack. It represents the largest contiguous unit of storage that can be accessed without moving the read/write heads.
- Track: A concentric ring on a single surface of a disk platter where data is magnetically recorded. Each track has a fixed capacity, typically measured in bytes.
- Block/Record: The fundamental unit of data written to a track. On Count Key Data (CKD) devices, blocks are variable in length and include control information (count area, key area, data area).
- Count Key Data (CKD) Architecture: The dominant architecture for mainframe DASD, where data blocks are self-describing, containing address, length, and optional key information, allowing for variable-length records.
- Physical vs. Logical Addressing: While physical geometry refers to the actual hardware layout, z/OS and its storage management components translate logical addresses (e.g., dataset extent definitions) into physical cylinder/track/record addresses.
- Impact on I/O Performance: The spatial layout directly influences head movement (seek time) and rotational latency, making efficient data placement crucial for optimal I/O performance.

Use Cases

- Dataset Allocation: When defining datasets in JCL, the SPACE parameter uses geometry units (CYL for cylinders, TRK for tracks) to specify the primary and secondary allocation sizes on a DASD volume.
- Performance Tuning: System programmers and storage administrators strategically place frequently accessed datasets on specific cylinders or volumes to minimize seek times and optimize I/O paths.
- Storage Management: Understanding DASD geometry is essential for managing free space, preventing fragmentation, and planning for volume reorganization or defragmentation.
- Backup and Recovery: Knowledge of data placement and geometry helps in planning efficient backup strategies and understanding the physical layout for recovery operations.
- Problem Determination: Analyzing I/O errors or performance bottlenecks often requires examining the physical location of data on DASD volumes relative to their geometry.

Related Concepts

DASD geometry is intrinsically linked to DASD (Direct Access Storage Device) hardware itself, as it describes the physical structure. It is directly utilized by JCL (Job Control Language) through the SPACE parameter for allocating datasets. The Volume Table of Contents (VTOC) on each DASD volume records the extent information for datasets, defining their location using cylinder and track addresses. Storage Management Subsystem (SMS) abstracts much of the manual geometry management, but its underlying functions still rely on these concepts for optimal data placement and performance. Furthermore, the efficiency of I/O operations is heavily dependent on how data is laid out according to the DASD geometry.

Best Practices:

Allocate in Cylinders for Large Datasets: Whenever possible, allocate large or critical datasets in CYL units rather than TRK to ensure better contiguity, reduce fragmentation, and minimize the number of dataset extents.
Minimize Extents: Aim for datasets to be allocated in as few extents as possible (ideally one primary extent) to reduce I/O overhead associated with seeking across multiple non-contiguous areas.
Leverage SMS: Utilize the Storage Management Subsystem (SMS) to automate and optimize dataset placement based on performance, availability, and space requirements, abstracting the complexities of manual geometry management.
Monitor DASD Space and Fragmentation: Regularly monitor DASD volumes for free space availability and fragmentation. Use utilities like ICKDSF or DFSMS tools to manage and reorganize volumes as needed.
Consider Data Set Placement: For highly active datasets, consider placing them on volumes with less contention or on faster storage tiers, though modern virtualized storage often handles this transparently.