File Sharing - Multiple access

Enhanced Definition

In the z/OS environment, file sharing with multiple access refers to the capability for multiple jobs, users, or address spaces to concurrently read from or write to the same dataset. This critical function requires robust mechanisms to ensure data integrity, prevent corruption, and manage contention across various applications and systems.

Key Characteristics

- Dataset Types: Applicable to various z/OS dataset organizations, including sequential, VSAM (KSDS, ESDS, RRDS), and partitioned datasets (PDS and PDSE).
- ENQ/DEQ Services: z/OS uses ENQ (Enqueue) and DEQ (Dequeue) services to manage access to resources, including datasets, allowing for shared (read) or exclusive (write) control.
- Shared DASD: Often implemented on Shared DASD (Direct Access Storage Device) configurations, where multiple z/OS systems in a sysplex can access the same physical storage volumes.
- Data Integrity: Mechanisms like serialization, locking, and VSAM Record Level Sharing (RLS) are employed to maintain data consistency and prevent concurrent updates from corrupting data.
- JCL DISP Parameter: The DISP parameter in Job Control Language (JCL) explicitly declares the sharing intent for a dataset, such as DISP=(SHR,...) for shared access or DISP=(OLD,...) for exclusive access.
- Access Method Dependence: The specific capabilities and limitations for multiple access can vary depending on the access method used (e.g., QSAM, BSAM, VSAM).

Use Cases

- Batch/Online Coexistence: A CICS online transaction processing system updating a DB2 database while a batch reporting job concurrently reads data from the same database datasets.
- Shared Libraries: Multiple developers or batch jobs accessing common load libraries (PDSE) or source code libraries (PDS) for program execution or compilation.
- Concurrent Reporting: Several reporting applications simultaneously reading from a large master transaction file to generate different analytical reports.
- Database Access: Multiple applications (e.g., CICS, IMS, batch) accessing the same DB2 or IMS database datasets, with the database manager handling concurrency control.
- Spool File Management: Multiple applications writing SYSOUT to JES spool datasets, which are then accessed by print utilities or viewing tools.

Related Concepts

File sharing is a cornerstone of z/OS resource management, data integrity, and sysplex operations. It heavily relies on serialization services (ENQ/DEQ) to coordinate access, especially in a Shared DASD environment. For advanced sharing, VSAM RLS provides record-level locking, while database management systems like DB2 and IMS offer sophisticated concurrency control for their managed datasets. The JCL DISP parameter is the primary job -level declaration of sharing intent.

Best Practices:

Use DISP=SHR for Read-Only: Always specify DISP=SHR in JCL for datasets that are only being read to maximize concurrency and avoid unnecessary contention.
Minimize DISP=OLD Duration: When exclusive access (DISP=OLD) is required for updates, ensure the dataset is held for the shortest possible duration to reduce contention and improve throughput.
Leverage Database Managers: For complex shared data with high concurrency requirements, utilize DB2 or IMS, as they provide robust locking, concurrency control, and recovery mechanisms.
Implement VSAM RLS: For high-performance, concurrent sharing of VSAM datasets across a sysplex, configure and utilize VSAM Record Level Sharing (RLS) to enable record-level locking.
Monitor Contention: Regularly monitor ENQ contention using tools like RMF or OMEGAMON to identify and resolve bottlenecks caused by excessive exclusive dataset access.
Understand PDS vs. PDSE: Be aware that PDS provides member-level serialization, while PDSE offers more advanced concurrent write access to different members, making it generally preferred for shared libraries.