Direct Access - Non-sequential storage
Direct access storage refers to the ability to retrieve or store data directly at any specific location on a storage device without having to read through preceding data. In the mainframe context, this primarily applies to **Direct Access Storage Devices (DASD)**, which allow non-sequential, random access to data records based on their physical or logical address. It is the cornerstone for most online and database applications on z/OS. Direct access, in the context of mainframe systems, refers to a method of data organization and retrieval where individual records within a dataset can be accessed, read, or updated directly and randomly, without the need to process preceding records. This non-sequential capability is fundamental for rapid data retrieval and modification, especially in online transaction processing and database environments.
Key Characteristics
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- Random Access Capability: Data can be accessed directly by its address (e.g., cylinder, track, record number, or relative byte address), allowing for rapid retrieval or update of individual records without scanning the entire file.
- Addressability: Data records are stored at specific, addressable locations on the storage medium, enabling precise targeting for read/write operations. This is often managed by the operating system and access methods.
- In-Place Updates: Records can be updated directly at their stored location without requiring the entire file to be rewritten, which is crucial for transactional systems and dynamic data.
- Persistent Storage: Data remains available even after the application or system using it has terminated, making it suitable for databases, system files, and user data that needs to be retained.
- Device Type: Predominantly associated with DASD (e.g., IBM 3390 series or its emulated counterparts), which are block-oriented devices managed by the z/OS operating system.
Use Cases
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- Database Systems: Core for relational databases like DB2 and hierarchical/network databases like IMS DB, where individual records need to be quickly accessed, inserted, updated, or deleted based on keys or pointers.
- Indexed Files: Used by access methods like VSAM KSDS (Key-Sequenced Data Set) and older ISAM (Indexed Sequential Access Method) to provide fast retrieval of records based on a primary key, essential for many business applications.
- Online Transaction Processing (OLTP): Critical for systems like CICS, where many users concurrently access and update small amounts of data rapidly, requiring immediate data availability and integrity.
- System Libraries: Partitioned Data Sets (PDS) and Partitioned Data Set Extended (PDSE), which store programs, JCL, load modules, and other system components, rely on direct access to locate specific members quickly.
- Application Data Files: Many custom COBOL or PL/I applications use direct access methods (e.g., BDAM or VSAM RRDS/ESDS) for files where records are accessed by relative record number or direct address.
Related Concepts
Direct access is fundamental to the concept of DASD (Direct Access Storage Devices), which are the primary storage medium for most mainframe data. It contrasts sharply with sequential access, typically associated with tape drives, where data must be read in the order it was written. Various access methods like VSAM (Virtual Storage Access Method), BDAM (Basic Direct Access Method), and ISAM are built upon the capability of direct access to manage data sets efficiently. It enables the functionality of databases (DB2, IMS) and transaction managers (CICS) by providing the underlying mechanism for rapid, non-sequential data manipulation, making it indispensable for modern enterprise computing on z/OS.