Main storage databases (MSDBs)

MSDBs contain only root segments. Each segment is like a database record, because the segment contains all of the information about a particular subject.

In a DL/I hierarchy, a database record is made up of a root segment and all its dependents. For example, in the medical hierarchy, a particular PATIENT segment and all the segments underneath that PATIENT segment comprise the database record for that patient. In an MSDB, the segment is the whole database record. The database record contains only the fields that the segment contains. MSDB segments are fixed length.

The two kinds of MSDBs are terminal related and non-terminal related. In terminal-related MSDBs, each segment is owned by one logical terminal. The segment that is owned can be updated only by that terminal. Related MSDBs can be fixed or dynamic. You can add segments to and delete segments from dynamic related MSDBs. You cannot add segments to or delete segments from fixed related MSDBs.

In the second kind of MSDB, called non-terminal related (or nonrelated) MSDBs, the segments are not owned by logical terminals.