Considerations

Control-M/Tape can use Fast Positioning for a data set even if other data sets on the tape were created without Fast Positioning (meaning, a tape can contain any combination of data sets created with and without Fast Positioning).

Control-M/Tape receives the position of a data set from the operating system and records the position in the Media Database only when you create or rewrite the data set, not when you read it. Therefore, if you create a data set without recording its position (for example, with the expression DO FASTPOS=NO), Control-M/Tape cannot use Fast Positioning for that data set. Even if you later set DO FASTPOS to YES, and then read the data set, Control-M/Tape cannot use Fast Positioning for it.

If Control-M/Tape determines that the requested Fast Positioning is not appropriate (for example, the JCL includes the expression DISP=MOD) and the application does not have its own Fast Positioning mechanism, Control-M/Tape allows the operating system to use conventional sequential search positioning. In such a case, Control-M/Tape does not display or log an error message.

When Control-M/Tape passes a Fast Positioning request to the operating system, the operating system examines the request and determines whether Fast Positioning is worthwhile. If the operating system determines that Fast Positioning is not worthwhile (for example, if the requested block ID is near the beginning of the tape), the operating system uses the conventional sequential search mechanism instead. This means that a Control-M/Tape request for Fast Positioning never increases positioning time. It either decreases it or leaves it unchanged.

Because Fast Positioning can dramatically speed up jobs, it is recommended that you include the expression DO FASTPOS=YES in Control-M/Tape rules for all data sets that you write.