Fast Positioning a Tape

The Fast Positioning facility cuts the time required to position a tape to a data set. With Fast Positioning, the operating system does not sequentially search the tape for a data set. Instead, the operating system passes the block ID of the data set, which is the location of the data set on the tape, to the tape device. Using this block ID, the tape device positions the tape at the beginning of the data set. If a tape contains many data sets or large data sets, fast positioning can be up to14 times faster than searching sequentially for the data set.

Control-M/Tape greatly simplifies implementation of Fast Positioning. Implementation can be accomplished by defining a single Control-M/Tape rule. Control-M/Tape implementation of Fast Positioning is invisible to your applications, and requires no changes to the programs or JCL.

Note: Control-M/Tape does not directly Fast Position a tape. It instead requests the service from the operating system.

Fast Positioning of Control-M/Tape works as follows:

1. At the time of data set creation, Control-M/Tape retrieves the block ID of the data set from the operating system, and stores it in the Control-M/Tape Media Database. The block ID identifies the data set location.
2. During subsequent access of the data set, Control-M/Tape retrieves the block ID from the Media Database, and passes it to the operating system, that then fast positions the tape to the specific location.

Control-M/Tape can apply Fast Positioning to the following data set access situations:

• Reading a data set
• Rewriting a data set.
• Adding a data set to the end of a tape. (In this case, Control-M/Tape uses the block ID at the end of the tape volume.)