DL/I considerations

Consider the following things when designing DL/I databases.

• Use DL/I path calls wherever possible to reduce the DL/I path length and program execution time.
• Use qualified DL/I calls wherever possible to minimize the number of calls required to obtain the desired segment.
• Do not issue a GN call to the I/O PCB if the application program always retrieves single-segment input messages.
• Ensure that the application program returns to issue another GU to the I/O PCB on completion. This allows processing of another transaction of the same type if one exists on the input queue. This avoids the overhead of termination and rescheduling processing.
• When output is sent to several TP-PCBs, define multiple alternate PCBs rather than using the CHNG call.
• Select HDAM (or PHDAM, if using HALDBs) as the organization for a database wherever possible, because, when well tuned, HDAM (or PHDAM) generally results in shorter path lengths and fewer I/Os per DL/I call.

In assessing the characteristics of transaction processing and the interaction of application programs and databases, be aware of the performance design considerations presented in IMS Version 15.5 Database Administration and IMS Version 15.5 Application Programming.