Areas and the DEDB format

A DEDB can use multiple data sets, called areas, with each area containing the entire data structure.

The physical format of DEDBs makes the data they contain more readily available. In a hierarchical IMS database that does not use areas, the logical data structure is spread across the entire database. If multiple data sets are used, the data structure is broken up on a segment basis.

Each area in a DEDB is a VSAM data set. A DEDB record (a root and its dependent segments) does not span areas. A DEDB can be divided into as many as 9999 such areas. This organization is transparent to the application program.

The maximum size of a DEDB area is 4 GB. The maximum number of areas per database is 9999; thus, the maximum size of a DEDB database is 39996 GB (approximately 40 TB).

Run the DEDB Initialization utility (DBFUMIN0) to format each area to DBD specifications. Root-addressable and independent-overflow parts are allocated accordingly. The space left in the VSAM cluster is reserved for the sequential-dependent part. Up to 9999 areas can be specified in one utility run; however, the area initializations are serialized. After you run the utility, check the statistical information report against the space calculation results.

If the total number of DEDB areas after the ADDAREA function exceeds 9999 DEDB areas, the DEDB Alter utility terminates.

IMS does not enforce a limit on the number of area data sets that can be open at the same time by multiple DEDB databases. However, the resources available at your installation and the consumption of those resources by both your IMS configuration and the other z/OS® subsystems that your installation might be running, such as Db2 for z/OS, could potentially limit the number of area data sets that you can open.

For area data sets, one of the resources that could become constrained with a very large number of open data sets is storage in the extended common service area (ECSA) and the extended private storage (EPVT).

The randomizing module is used to determine which records are placed in each area. Because of the area concept, larger databases can exceed the limitation of 2³² bytes for a single VSAM data set. Each area can have its own space management parameters. You can choose these parameters according to the message volume, which can vary from area to area. DEDB areas can be allocated on different volume types.

Initialization, reorganization, and recovery of DEDBs are done on an area basis. Resource allocation is done at the control interval (CI) level. Multiple programs, optionally together with one online utility, can access an area concurrently within a database, as long as they are using different CIs. CI sizes can be 512 bytes, 1 K, 2 K, 4 K, and up to 28 K in 4 K increments. The media manager and Integrated Catalog Facility catalog of Data Facility Storage Management Subsystem (DFSMS) are required.