Available Frame

Enhanced Definition

An **available frame** in z/OS refers to a unit of real storage (main memory) that is currently unused and unassigned to any virtual storage page. It represents a readily accessible block of physical memory that the system can allocate to satisfy a demand for real storage, typically when a virtual storage page needs to be brought into main memory.

Key Characteristics

- Unit of Real Storage: An available frame is a physical block of main memory, typically 4KB in size, corresponding to the standard page size in z/OS. Larger frames (1MB or 2GB) also exist but are managed differently.
- Unassigned Status: It is not currently holding any active virtual storage page content and is therefore available for immediate allocation by the system.
- Managed by RSM: The z/OS Real Storage Manager (RSM) is responsible for maintaining a pool of available frames, often referred to as the *available frame queue*.
- Dynamic Allocation: These frames are dynamically allocated by the system when a virtual storage page needs to be materialized in real storage, such as during a page-in operation.
- Contributes to System Performance: A healthy supply of available frames is crucial for efficient system operation, reducing the need for page-out operations to free up frames when demand for real storage is high.

Use Cases

- Page-in Operations: When a program references a virtual storage page that is not currently in real storage (a page fault), an available frame is acquired to load the page from auxiliary storage (DASD).
- New Storage Allocation: For newly allocated virtual storage areas (e.g., for a new task, data space, or program load), an available frame is used when the corresponding virtual pages are first referenced.
- System Buffer Pools: When system components or applications require new buffer space in real storage, available frames are used to back these allocations.
- Dynamic Real Storage Management: The system continuously manages the available frame queue, attempting to maintain a minimum number of frames to ensure responsiveness and avoid excessive paging.

Related Concepts

An available frame is fundamental to virtual storage management in z/OS. It is a physical resource that virtual storage pages are mapped to. The Real Storage Manager (RSM) is the z/OS component responsible for managing all real storage frames, including maintaining the pool of available frames and performing paging operations (page-in/page-out). When a page fault occurs, the RSM attempts to find an available frame to bring the required page from auxiliary storage into real memory. The status of all real storage frames, including their availability, is tracked in the Page Frame Table (PFT).

Best Practices:

Monitor Available Frame Count: Regularly monitor the number of available frames using tools like RMF or SMF data. A consistently low count indicates real storage constraint and potential performance degradation.
Optimize Real Storage Usage: Identify and tune applications or system components that are excessive real storage consumers. Use techniques like page fixing judiciously, as fixed pages reduce the available frame pool.
Ensure Sufficient Real Storage: Provision adequate physical memory (real storage) to support the anticipated workload, ensuring a healthy buffer of available frames.
Analyze Paging Activity: High paging rates, especially page-outs, can indicate a shortage of available frames. Investigate the cause and consider increasing real storage or optimizing workload distribution.
Tune RSM Parameters: Adjust z/OS system parameters related to RSM (e.g., RSMFIX, RSMREG in IEASYSxx) to influence how real storage is managed and