Data space for data tables

The data table records are stored in an MVS™ data space, whether the data table is to be shared by more than one region or not. A separate data space is used for each CICS® region. The data space, named DFHDT001, is obtained when the first file that is defined as a data table is opened in the region. It is used by all CICS data tables that are owned by that region, and it is retained until the shutdown of CICS in the region.

The data space for each CICS region has a maximum size of 2GB. The MVS exit IEFUSI, which can be used to control the total amount of data space that a given address space may own, can reduce the maximum size to less than 2GB. Within this limit, CICS allocates data space storage in units of 16MB, and then sub-allocates this storage to the data tables in increments of 128KB. If a new storage increment is needed for a data table, but all the existing data space storage is already allocated to tables, CICS attempts to extend the data space by 16MB. If CICS is unable to extend the data space, either because the data space has reached the maximum size of 2GB, or because the total data space size set by the installation's IEFUSI exit has been reached, then CICS notes that the data space is now full.

If a data space is full, any shared data table requests which need additional storage are failed because of insufficient storage. For a CICS-maintained data table, this means that any future reads for the affected records (including any approximate reads near to that key) must access the VSAM data set, using function shipping if the request is not issued from the file owning region. For a user-maintained data table, this means that the record cannot be written to the table. Application programming for data tables has information about the response returned in this situation.

CICS does not provide a facility for viewing the current size of the overall data space. However, the CICS file control statistics can give an accurate indication of the storage allocated and used for each data table. In particular, the field A17DTALD contains the amount of data space storage (in KB) that is currently allocated to the table.

The data space storage that is used by the data table is freed when the file is closed in the FOR. This storage is made available for reuse in such a way that the integrity of any AOR that was using the data table is protected.

Related concepts
Introduction to shared data tables
The concept of shared data tables
Description of data tables
Data table sharing environment
Source data set for data tables
Global user exits for data tables
Benefits of shared data tables
Shared data table services and remote file access
How a data table is shared
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