What happens when you use DUMMY parameter for a dataset

When the DUMMY parameter is specified for a dataset, no disk or tape resources are allocated to that dataset, and no I/O operations are performed against it.

During batch job testing, I frequently use the DUMMY parameter for output datasets that would otherwise contain millions of records and are not required for validation. By eliminating the creation of these unnecessary output files, the job avoids the associated I/O overhead, resulting in shorter execution times of the job

The following example illustrates the behaviour of DUMMY parameter.

Sample job with the DUMMY parameter specified for the output file

//STEP1 EXEC PGM=SMF30ASM    
//STEPLIB  DD DSN=USERID.LOAD,DISP=SHR        
//SYSPRINT DD SYSOUT=*                                          
//SMF30IN DD DISP=SHR,DSN=USERID.WEEKLY.SMF30 
//OUT     DD DUMMY,                   
//        DISP=(NEW,CATLG,DELETE),                          
//        SPACE=(CYL,(50,50,)),
//        DCB=(LRECL=140,RECFM=FB)  

                               
The "EXCP Statistics" section of the JESYSMSG in the job log does not contain an entry for DDNAME OUT. This indicates that no I/O operations were performed against DDNAME OUT.

EXCP Statistics
===============
DDNAME   CC# Unit EXCP Count
STEPLIB      914F         15
SMF30IN   +2 9087      48001
SMF30IN      9186      48001
SMF30IN   +3 9284      23996
SMF30IN   +1 9080      26856
SMF30IN   +1 9187      48001

The above job with the output file defined without the DUMMY parameter
======================================================================
//STEP1 EXEC PGM=SMF30ASM                                
//STEPLIB  DD DSN=USERID.LOAD,DISP=SHR       
//SYSPRINT DD SYSOUT=*                                          
//SMF30IN DD DISP=SHR,DSN=USERID.WEEKLY.SMF30 
//OUT     DD DSN=USERID.SMF30.ASM.OUT,           
//        DISP=(NEW,CATLG,DELETE),                          
//        SPACE=(CYL,(50,50,))              
//        DCB=(LRECL=140,RECFM=FB)  

                           
The "EXCP Statistics" section of the JESYSMSG in the job log shows the number of I/O operations performed against DDNAME OUT.    

EXCP Statistics
===============
DDNAME   CC# Unit EXCP Count
STEPLIB      914F         15
SMF30IN   +1 9187      48001
SMF30IN   +2 9087      48001
SMF30IN      9186      48001
SMF30IN   +3 9284      23996
SMF30IN   +1 9080      26856
OUT          9187      12898