HIDAM block structure
A HIDAM database is formed of two separate databases.
The HIDAM primary index database is a VSAM KSDS (Pointer Checker does not support ISAM), which is formed of records that contain the key of a root segment and a pointer to it. No segments exist in the HIDAM primary index database. The structure of HIDAM primary index is discussed in a later section.
The data segments are contained in the main database, which is an OSAM or VSAM ESDS. The structure of the blocks/CIs is slightly different, depending on which access method is used. It also varies depending on whether twin backward pointers or hierarchical backward pointers have been specified for the root segment. If backward pointers have not been specified, IMS keeps a RAP in the block/CI and maintains a LIFO chain of roots within the block/CI in association with the twin forward pointer or the hierarchical forward pointer. If backward pointers have been specified, then the RAP is not needed since the chain is maintained through the twin forward or hierarchical forward pointers.
The following figures show a HIDAM block structure without backward pointers in the root and with backward pointers in the root. More information about these block structures follows the figures.
FSEAP | FSEAP as described in a previous section. |
|---|---|
RAP | RAP area as described in a previous section. |
Segments | Segments as described in a previous section. Segments may not necessarily be contiguous as they may be interspersed with free space (including slack bytes). |
FSEs | FSEs as described in a previous section. FSEs and free space may be interspersed with segments. |
Free space | Free space following an FSE as described in a previous section. FSEs and free space may be interspersed with segments. |
VSAM RDF | Standard VSAM record definition field control blocks, which describe the size of the logical record. HIDAM main databases have only one logical record in each CI. |
VSAM CIDF | Standard VSAM control interval definition field control blocks, which describe the size of the control interval. |
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