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Only one DO STACK statement can be specified per rule definition.
Stacking directs a data set dynamically at the time of its creation to a volume that already contains one or more data sets but has space available. This enables more complete utilization of free space on active tapes in your library.
If a rule contains the expression DO STACK=Y, Control-M/Tape implements real-time (Dynamic) data set stacking (provided environmental conditions permit stacking to be performed).
If a rule contains the expression DO STACK=N, Control-M/Tape does not stack the data set, unless a more specific, or higher priority, rule that applies to the data set contains the expression DO STACK=Y.
If a rule contains the expression DO STACK=N and SCOPE DSN, Control-M/Tape may later stack other data sets on the same volume as the current data set. In other words, the current data set is the first, but not necessarily the only, data set on the volume.
If a rule contains the expression DO STACK=N and SCOPE VOL, Control-M/Tape does not stack other data sets on the same volume as the current data set. In other words, the current data set is the first and only data set on the volume.
If a rule does not contain a DO STACK statement, Control-M/Tape does not stack the data set unless another rule that applies to the data set contains the expression DO STACK=Y.
Note: Using the expression DO STACK=N with SCOPE DSN is equivalent to the expression DO STACK=N prior to version 5.1.4. Under JES3, the expression DO STACK=Y can be specified only with selection criteria ON DATASET, ON JOBNAME, and/or ON USERID.
To stack data sets, Control-M/Tape tracks and calculates the average size of each data set (on the basis of the sizes of the various versions of the data set). This information is stored in the Stacking Database. Based on the average size, Control-M/Tape estimates whether a data set will fit on a specific volume.
When a data set is written to a volume, Control-M/Tape calculates the remaining space on the volume. This calculation is based on the volume’s capacity and the sizes of the data sets that the volume contains. By calculating the remaining space on a volume, Control-M/Tape can determine whether it is possible to stack a data set.
Using these calculations, when a job that creates a data set is submitted, Control-M/Tape checks if the data set can be stacked. If so, Control-M/Tape modifies the job’s JCL to direct the data set to a suitable volume.
For additional information about stacking, see the data set stacking chapter of the Control-M/Tape Implementation Guide.
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