File Allocation - Space Assignment

Enhanced Definition

File allocation, in the z/OS context, refers to the process of reserving and assigning specific storage space on a Direct Access Storage Device (DASD) for a dataset. This ensures that a program or utility has dedicated physical storage available before it attempts to write or process data, preventing runtime errors related to insufficient space. It is a fundamental aspect of data management on the mainframe.

Key Characteristics

- Primary and Secondary Allocation: Datasets are typically allocated with a PRIMARY space quantity and an optional SECONDARY quantity. The primary space is allocated initially, and the secondary space is allocated incrementally if the primary space is exhausted.
- Units of Allocation: Space can be specified in CYLINDERS (tracks per cylinder), TRACKS, or BLOCKS (average record length). Cylinders are often preferred for larger datasets due to efficiency.
- Contiguous Space: While z/OS generally allows datasets to be non-contiguous, the CONTIGUOUS parameter can be specified to request that all primary space be allocated in one contiguous extent for performance-critical sequential datasets.
- Volume Selection: Space can be allocated on a specific DASD volume (VOL=SER=volser) or on any available volume within a generic unit type (UNIT=SYSDA) or storage group (via SMS).
- Dataset Types: Allocation parameters vary slightly depending on the dataset organization (e.g., Physical Sequential (PS), Partitioned Dataset (PDS/PDSE), VSAM).
- Dynamic vs. Static: Allocation can be explicitly defined in Job Control Language (JCL) (static) or requested by an application program at runtime using dynamic allocation services (SVC 99).

Use Cases

- JCL DD Statement: The most common method for allocating new datasets (e.g., DSORG=PS, DSORG=PO, DSORG=DA) or specifying space for temporary work files within batch jobs.
- IDCAMS DEFINE Command: Used specifically for allocating and defining VSAM (Virtual Storage Access Method) datasets (e.g., DEFINE CLUSTER, DEFINE AIX).
- TSO ALLOCATE Command: Allows interactive users to allocate datasets from the Time Sharing Option (TSO) command line for personal use, testing, or temporary storage.
- Dynamic Allocation in Programs: Application programs (e.g., COBOL, PL/I, Assembler) can dynamically request and allocate datasets at runtime, often for temporary files or when dataset names are determined programmatically.
- Utility Programs: Many z/OS utilities (e.g., IEBGENER, DFSORT) allocate work datasets or output datasets as part of their processing.

Related Concepts

File allocation is intrinsically linked to JCL (Job Control Language), which provides the syntax (SPACE parameter in DD statements) to define allocation requirements. It directly interacts with DASD (Direct Access Storage Device) to reserve physical storage. The Dataset Organization (PS, PDS, VSAM) dictates specific allocation considerations. Furthermore, it's a core component of Storage Management strategies, especially when integrated with DFSMS (Data Facility Storage Management Subsystem), which automates allocation decisions based on storage classes, management classes, and storage groups, abstracting the physical details from the user. Once allocated, datasets are often Cataloged to allow subsequent access without specifying full volume and unit information.

Best Practices:

Estimate Space Accurately: Avoid over-allocating (wasting valuable DASD space) and under-allocating (leading to B37 or D37 abends). Use utilities like DFSMSrmm or ISPF 3.4 to monitor dataset growth.
Utilize SECONDARY Allocation: Always specify a SECONDARY quantity to allow for dataset growth without requiring re-allocation or causing abends, especially for datasets that might expand.
Use RLSE Parameter: For sequential datasets, specify SPACE=(TRK,(primary,secondary),RLSE) to release any unused allocated space back to the system when the dataset is closed, optimizing DASD utilization.
Leverage DFSMS: Implement DFSMS to automate and standardize allocation decisions, ensuring datasets are placed on appropriate storage devices based on performance, availability, and cost requirements.
Monitor DASD Fragmentation: While z/OS handles fragmentation, frequent small allocations and de-allocations can lead to inefficient space usage. Periodically reorganize fragmented datasets or volumes if performance becomes an issue.
Consider CONTIGUOUS Judiciously: Only request CONTIGUOUS allocation for datasets where sequential read performance is absolutely critical, as it can be harder to satisfy and may lead to allocation failures on fragmented volumes.