Extended addressability support

You can access VSAM data sets that are defined with the extended addressability attribute, use those VSAM data sets in COBOL programs without COBOL source changes, and maintain compatibility with previous versions of COBOL.

With extended addressability support, you can define larger VSAM data sets outside of COBOL. The 4 GB VSAM architectural limit for data set size imposed by using the 4-byte field for the relative byte address (RBA) is eliminated.

To use the extended addressability, the VSAM data set must be Storage Management Subsystem (SMS)-managed and be defined as extended format. The size limit for a VSAM data set is determined in either of the following ways:
  • Control Interval (CI) size multiplied by 4 GB
  • Volume size multiplied by 59
For example, a 4 KB CI size yields a maximum data set size of 16 TB, and a 32 KB CI size yields a maximum data set size of 128 TB. A 4 KB CI size is preferred by many applications for performance reasons. For extended-format data sets that grow beyond 4 GB, the processing time does not increase.

Extended addressability is also supported for programs compiled with earlier versions: VS COBOL II programs compiled with RES and any later compilers.

Extended addressability and extended format are not the same concept. Extended format is a prerequisite for extended addressability. Extended format is a technique that affects the way of storing count key data (CKD) in a 3390/3380 logical track. Extended format implements data striping and increases the performance and the reliability of an I/O operation. If a data set is allocated as an extended-format data set, 32 bytes are added to each physical block.

Restriction: Extended addressability was introduced for KSDS data sets in DFSMS/MVS V1.3. Since DFSMS/MVS V1.4, extended addressability is supported in record level sharing (RLS). With DFSMS/MVS V1.5, support for extended addressability is extended to all other VSAM record organizations.

Related tasks  
z/OS® DFSMS: Using Data Sets