Expand - Increasing size

Enhanced Definition

In the mainframe context, "expand" primarily refers to increasing the allocated size or capacity of a resource, such as a dataset, a storage area, or a system component, to accommodate more data or workload. This often involves acquiring additional storage extents on `DASD` or increasing defined virtual storage limits.

Key Characteristics

- Extent-based Allocation: For DASD datasets, expansion typically means allocating additional extents on a volume, up to a maximum number of extents per dataset (e.g., 16 for non-VSAM on a single volume, or 255 for VSAM and SMS-managed datasets).
- Dynamic vs. Explicit: Expansion can occur dynamically (e.g., a VSAM dataset extending automatically as data is added) or require explicit action (e.g., modifying JCL SPACE parameters for a sequential dataset or PDS).
- Virtual Storage Expansion: Refers to increasing the amount of virtual storage available to an address space, region (e.g., CICS, IMS), or system component, often managed by z/OS services.
- Dataset Attributes: The ability and parameters for expansion are often governed by dataset attributes defined in JCL (SPACE=(TRK,(primary,secondary))) or IDCAMS DEFINE statements for VSAM datasets (CYLINDERS(primary secondary)).
- System Limits: Expansion is always constrained by available physical resources (e.g., DASD space, real memory) and system-defined limits, such as maximum extents or region sizes.
- Capacity on Demand (CoD): At the hardware level, "expand" can mean activating pre-installed, but inactive, processors or memory via Capacity on Demand features on zSystems.

Use Cases

- Dataset Growth: A sequential dataset or PDS/PDSE reaching its primary allocation limit and requiring additional extents to store more records, triggered by a write operation.
- VSAM File Extension: A VSAM KSDS or ESDS automatically extending its allocated CIs (Control Intervals) and CAs (Control Areas) as new records are inserted, provided its DEFINE parameters allow for secondary allocation.
- CICS Region Storage: Increasing the DSA (Dynamic Storage Area) or other storage pools within a CICS region to handle more transactions, larger working sets, or increased concurrent users.
- JCL SPACE Parameter Adjustment: Modifying the secondary allocation in a JCL DD statement (e.g., SPACE=(CYL,(10,5))) for a new or existing dataset to allow for more growth without manual intervention.
- IMS Database Expansion: Adding new data sets to an IMS database or increasing the size of existing ones to accommodate more segments and maintain performance.

Related Concepts

Expansion is intrinsically linked to storage management and resource allocation in z/OS. For DASD datasets, it relies on the Volume Table of Contents (VTOC) to track extents and interacts with Data Facility Product (DFP) components for space allocation. It's also closely related to virtual storage management and the concept of address spaces, where regions can be expanded to accommodate application needs. The ability to expand is a critical design consideration for scalability and high availability of mainframe applications and systems.