DASD - Direct Access Storage Device

Enhanced Definition

DASD, or Direct Access Storage Device, is a fundamental type of non-volatile storage in IBM mainframe environments, allowing data to be accessed directly and randomly without reading through preceding data. It serves as the primary storage medium for most online applications, databases, and system files on z/OS.

Key Characteristics

- Direct Access Capability: Unlike sequential devices (like tape), DASD allows immediate access to any data record based on its address, significantly improving retrieval times for random access workloads.
- Random Read/Write: Supports efficient random read and write operations, which is crucial for transactional systems, databases (e.g., DB2, IMS), and interactive applications.
- Persistent Storage: Data stored on DASD remains intact even when the system is powered off, providing long-term data retention.
- Block-Oriented I/O: Data is typically read from and written to DASD in fixed or variable-size blocks (records), rather than byte by byte.
- High Capacity and Performance: Modern DASD subsystems (like IBM DS8000 series) offer massive storage capacities and high I/O throughput, often leveraging solid-state drives (SSDs) for performance-critical workloads.
- Shared Access: Multiple z/OS systems or LPARs can concurrently access the same DASD volumes, managed by mechanisms like GRS (Global Resource Serialization) or PPRC (Peer-to-Peer Remote Copy) for data integrity and availability.

Use Cases

- Database Storage: The primary storage for critical mainframe databases such as DB2, IMS, and VSAM (Virtual Storage Access Method) files, enabling rapid transaction processing.
- System Datasets: Stores essential z/OS system files, including the SYSRES (system residence) volume, PAGE and SWAP datasets, SMF (System Management Facilities) data, and system libraries (PROCLIB, LINKLIB).
- Application Data and Libraries: Holds application program libraries (LOADLIBs), source code libraries (PDS, PDSE), and various application data files used by COBOL, PL/I, and Assembler programs.
- JES Spooling: Used by JES2 and JES3 (Job Entry Subsystem) to spool job input, output, and intermediate work files, facilitating efficient job processing.
- Virtual Storage Paging: Provides the backing store for virtual memory pages, allowing z/OS to manage and extend the available memory for running applications.

Related Concepts

DASD volumes are the physical containers for datasets, which are the logical units of data storage on z/OS. The management of DASD space and dataset placement is largely handled by DFSMS (Data Facility Storage Management Subsystem), which uses Storage Classes, Management Classes, and Storage Groups to automate storage policies. JCL (Job Control Language) DD (Data Definition) statements are used by programs to define and allocate datasets on specific DASD volumes, or allow DFSMS to manage their allocation.

Best Practices:

Optimize Dataset Allocation: Allocate datasets with appropriate space parameters, block sizes, and organization (VSAM, PDS, sequential) to minimize wasted space and improve I/O performance.
Leverage DFSMS: Utilize DFSMS Storage Classes and Management Classes to automate storage placement, data migration, backup, and retention policies, ensuring efficient and policy-driven storage management.
Monitor I/O Performance: Regularly monitor DASD I/O performance metrics (e.g., using RMF, SMF) to identify bottlenecks, optimize dataset placement, and ensure acceptable application response times.
Capacity Planning: Conduct proactive capacity planning for DASD to anticipate future storage needs, prevent out-of-space conditions, and ensure continuous system operation.
Implement Robust Backup and Recovery: Establish comprehensive backup and recovery strategies for critical DASD volumes and datasets using tools like DFSMSdss or ADSM/TSM to protect against data loss and facilitate rapid recovery.