IBM Mainframe Forum

HI Friends,

I need a help.

How to dummy a data set?

Will dummying unused data sets(created but not used for a long time) reduce CPU and DASD Utilization? (I am sure that deleting unused data sets save space but I require without deleting but dummying)


If I dummy unused data sets, how it impacts the jobs that are running?

Need more informations about this.

Awaiting your answers.

Shabu.

Changing a data set DD name in a job to DD DUMMY has no impact on running jobs. Nor is there any impact on any other jobs that could be submitted in the future -- unless you go into that PDS member (or whatever) and change it as well.

A job might run slightly faster with DD DUMMY, since input files will immediately hit end-of-file on first read (COBOL file status 10 would be set, for example) while output files will not have any data placed in them. However, if the vast majority of the job CPU time is being spent in the program doing processing steps, then you might not see any difference in CPU time or run time by changing a job DD name to DUMMY.

Hello,

Will dummying unused data sets(created but not used for a long time)

What does this mean? Even if it is a "long time" what happens when some process does need it/them?

Is the process writing these or is it reading them?

I guess i'm trying to say that i'm not sure i understand the question exactly. . .

shabukumar wrote:...HI Friends,

I need a help.

How to dummy a data set?

Will dummying unused data sets(created but not used for a long time) reduce CPU and DASD Utilization? (I am sure that deleting unused data sets save space but I require without deleting but dummying)


If I dummy unused data sets, how it impacts the jobs that are running?

Need more informations about this.

Awaiting your answers.

Shabu.

• How to dummy a data set?
  
  Consult the JCL reference manual for your z/OS release.
• Will dummying unused data sets(created but not used for a long time) reduce CPU and DASD Utilization? (I am sure that deleting unused data sets save space but I require without deleting but dummying)
  
  There are two or maybe three parts to this question.
  • Will dummying unused data sets(created but not used for a long time) reduce CPU and DASD Utilization?
    
    This is not something to do without complete understanding of the purpose of the data. Your installation may have a legal requirement to retain this data, even if it appears no one looks at it.
    
    Specifying a dataset as DUMMY will certainly save storage, but its CPU impact may prove to be difficult to measure. You cannot DUMMY some classes of datasets; a PDS dataset used as a true PDS (by that I mean the program is an Assembler program using the STOW and FIND macros) cannot be dummied. No VSAM dataset can be dummied.
• If I dummy unused data sets, how it impacts the jobs that are running?
  
  You cannot do this.

All depends on the circumstances - dummying a dataset will just make the program hit end of file on the first read as Robert stated. But, if the program no longer needs that data then the program should be amended, at some stage, to remove references to the file(s). Once the program has been amended and put into production then the JCL can can be tidied up (the DDNAMES removed altogether). In the meanwhile, if you are dummying a file in a concatenation then read VERY CAREFULLY about that in the JCL manuals

Hi Steve,

If I dummy unused data sets, how it impacts the jobs that are running?

You cannot do this.

I think i've gotten lost on this one. . .

Possibly i misunderstand something, but why can this not be done?

A DUMMY specification is in the JCL. If the job is running, the JCL has been submitted and cannot be altered after the fact.

Hi Steve,

Yup, i read it the wrong way. . . I wasn't thinking of after the job was already running.

Been one of those weeks today

d

As has been stated many time, terminology is critical in IT -- your post is in JCL part of the Forum and you use a term "DUMMY" which has a very specific meaning in the context that this is an IBM Mainframe Forum.

For example, I find it hard to comprehend this text from your post:

shabukumar wrote:Will dummying unused data sets(created but not used for a long time) reduce CPU and DASD Utilization? (I am sure that deleting unused data sets save space but I require without deleting but dummying)

The DUMMY parameter specifies that no device is to be allocated to this DD statement and that no disposition processing or I/O is to be done. All other JCL parameters on a DUMMY DD statement have to be syntactically correct. Use the DUMMY parameter if you know you will not need a file in a job step. So you've not really "removed" the file-in-question from the system, it's still there. And as Dick indicates, if it's needed later -- by removing the file-in-question, are you doing some good or bad is business decision; using DUMMY, as such, does not play any role here.