Data In Virtual

DIV

Enhanced Definition

Data In Virtual (DIV) is an IBM z/OS access method that allows an application program to treat data residing on a Direct Access Storage Device (DASD) as if it were directly part of its virtual storage address space. It maps a portion of a DASD data set into virtual memory, enabling programs to access and manipulate the data using standard memory instructions rather than explicit I/O calls.

Key Characteristics

- Memory-Mapped I/O: DIV provides a form of memory-mapped I/O, where a range of virtual addresses is directly associated with a specific range of blocks on a DASD data set.
- Page-Oriented Access: Data is accessed in fixed-size pages (typically 4KB), mirroring how z/OS manages virtual storage and real memory.
- Implicit I/O: Reads and writes to the mapped virtual storage area are automatically translated into physical I/O operations by z/OS when a page is referenced or modified, making I/O transparent to the application.
- Data Set Support: Primarily used with VSAM Linear Data Sets (LDS) but can also be used with basic sequential data sets.
- Performance Optimization: Can significantly improve performance for applications requiring frequent, random access to large, persistent data structures by reducing explicit I/O overhead.
- System Services: Utilizes z/OS system services (DIVACCESS, DIVCLOSE, DIVMAC) to establish and manage the virtual mapping.

Use Cases

- Large Table Lookups: Efficiently accessing large, frequently referenced lookup tables or indexes stored on DASD without needing to load the entire structure into real memory.
- Scratchpad Areas: Providing a very large, temporary work area for applications that process substantial amounts of data, acting as an extended memory buffer beyond conventional virtual storage limits.
- Shared Read-Only Data: Facilitating the sharing of read-only data across multiple address spaces by mapping the same DASD data set into each program's virtual storage.
- Complex Data Structures: Managing complex, persistent data structures that exceed available real memory but benefit from direct, virtual storage-like access patterns.

Related Concepts

DIV is deeply integrated with the z/OS virtual storage management system, extending its principles to external storage. It offers an alternative to traditional access methods like BSAM, QSAM, or VSAM, providing a more direct, memory-centric way to interact with DASD. It often works in conjunction with data spaces or Hiperspaces to manage the mapped data efficiently, especially when dealing with very large datasets that require buffering or shared access across address spaces.

Best Practices:

Strategic Application: Use DIV judiciously for specific scenarios where its benefits (implicit I/O, large data handling) clearly outweigh the overhead of setup and management; it's not a general-purpose replacement for all I/O.
Page Alignment: Design data structures and access patterns to align with 4KB page boundaries to optimize I/O performance and minimize page faults.
Error Handling: Implement robust error checking for DIVACCESS and DIVCLOSE calls, as improper handling can lead to system abends or data integrity issues.
Performance Monitoring: Monitor system metrics related to DIV activity (e.g., page-in/page-out rates, CPU utilization) to ensure it's performing as expected and not creating bottlenecks.
Data Integrity and Concurrency: When modifying data via DIV, especially in shared environments, ensure proper serialization and locking mechanisms are in place to maintain data integrity and prevent concurrent update issues.