Extended Format

Enhanced Definition

Extended Format refers to a data set organization type in z/OS that allows for significantly larger data sets and enhanced capabilities compared to traditional formats. It is primarily used for VSAM and sequential data sets, enabling them to exceed the 4GB size limit and leverage advanced storage features like striping and compression.

Key Characteristics

- Increased Size Capacity: Allows data sets to grow beyond the traditional 4GB limit, supporting sizes up to 2TB for a single data set and even larger for striped data sets.
- SMS Managed: Requires the data set to be managed by SMS (Storage Management Subsystem); it must be assigned to an SMS data class, storage class, and management class.
- Enhanced Performance: Can be combined with data striping to distribute data across multiple volumes, improving I/O performance for large sequential reads/writes.
- Compression Support: Enables hardware compression for sequential data sets, reducing storage consumption and potentially improving I/O efficiency by transferring less data.
- Extended Addressability: Utilizes 64-bit addressing for data set extents, allowing for a much larger number of extents and overall data set size.
- JCL Specification: Specified in JCL using DSNTYPE=EXT on the DD statement for sequential data sets or via SMS data class attributes.

Use Cases

- Large Database Backups: Storing very large database backups (e.g., DB2, IMS) as sequential data sets that exceed 4GB.
- Data Warehousing: Managing massive input or output files for data analytics, ETL processes, or reporting that require significant storage capacity.
- High-Performance Logging: Creating high-volume transaction logs or audit trails that benefit from striping for improved write performance.
- Archival Storage: Archiving large volumes of historical data where compression is desired to minimize storage costs.
- Application Data Sets: Supporting applications that generate or consume extremely large sequential files, such as scientific simulations or financial models.

Related Concepts

Extended Format is intrinsically linked to SMS (Storage Management Subsystem), as it's a prerequisite for allocating and managing these data sets. It builds upon the capabilities introduced by Large Format data sets (which broke the 65,535 track limit but not the 4GB limit for non-VSAM) by further extending size and feature support. It often works in conjunction with DFSMSdss for efficient data management and DFSMSdfp for its underlying I/O services. It's a key enabler for modern z/OS environments dealing with ever-increasing data volumes.

Best Practices:

Leverage SMS Policies: Ensure appropriate SMS data classes are defined to automatically allocate Extended Format data sets, specifying attributes like compression and striping where beneficial.
Monitor Space Utilization: Regularly monitor the space utilization of Extended Format data sets, especially striped ones, to ensure balanced allocation and prevent performance bottlenecks.
Plan for Striping: When using data striping, ensure sufficient disk volumes are available and properly configured across different channels/controllers to maximize I/O parallelism.
Consider Compression: Evaluate the benefits of hardware compression for sequential Extended Format data sets; while saving space, it introduces CPU overhead during read/write operations.
Backup and Recovery Strategy: Develop a robust backup and recovery strategy using tools like DFSMSdss that are optimized for handling large and potentially striped Extended Format data sets.