ABCs of z/OS System Programming Volume 3 - IBM Redbooks

5.35 Allocating a data set
//NEWDATA DD DSN=FILE.SEQ1,
// DISP=(,CATLG),
// SPACE=(50,(5,5)),AVGREC=M,
// RECFM=VB,LRECL=80
Figure 5-35 Allocating a sequential data set
FILE.SEQ1
Creating and allocating data sets
Many times the words create and allocate, when applied to data sets, are used in MVS as
synonyms. However, they are not.
► To create (on DASD) means to assign a space in VTOC to be used for a data set
(sometimes create implies cataloging the data set). A data set is created in response to
the DD card DISP=NEW in JCL.
► To allocate means to establish a logical relationship between the request for the use of the
data set within the program (through the use of a DCB or ACB) and the data set itself in
the device where it is located.
Figure 5-35 shows an example of JCL used to create a data set in a system-managed
environment.
These are characteristics of the JCL in a system-managed environment:
► The LRECL and RECFM parameters are independent keywords. This makes it easier to
override individual attributes that are assigned default values by the data class.
► In the example, the SPACE parameter is coded with the average number of bytes per
record (50), and the number of records required for the primary data set allocation (5 M)
and secondary data set allocation (5 M). These are the values that the system uses to
calculate the least number of tracks required for the space allocation.
Chapter 5. System-managed storage 297