IBM Mainframe Forum

Hello,

I have created GDG as below:

//IDCAMS EXEC PGM=IDCAMS,REGION=512K
//SYSPRINT DD SYSOUT=*
//SYSIN DD *
DEFINE GDG( -
NAME(TESTD.TAPE.TODISK)-
LIMIT(7) NOEMPTY SCRATCH)
/*
//

I used in my environment and its work fine.
But I have noticed online as:
Before using GDG , We need to create GDG index and model.
What I have created above? Is it model? If yes, what should be the index and how would it use?

thanks

You have NOT "created" a GDG using the posted JCL. You have defined a GDG base. Terminology is critical in IT, where similar terms may mean very different things. With a GDG, there can be a generation, or a version, or the GDG base, or the model DSCB. Your terms "index" and "model" are not typically used with a GDG, hence are meaningless in your post. Please research and find out exactly what you are asking, then rephrase your question in terms that make sense.

Mr. Sample is correct: proper terminology is very important. However, I think he may be nitpicking here: most people accept "GDG base" and "GDG index" as being equivalent.

However, model DSCBs (or perhaps "model datasets" might be better, though technically incorrect) are more confusing.

There are two types of model DSCBs:

• General purpose model DSCBs. These are cataloged datasets that are typically allocated with 0 tracks, are cataloged, and are referenced in JCL as DCB=dataset. Many shops had a few "stock" general purpose model DSCBs, like for example, FB80800 defined, that could be used in composite DCB parameters like
  
  DCB=(FB80800,RECFM=..,LRECL=...,BLKSIZE=...)
  
  when allocating GDG datasets.
• Model DSCBs for GDG datasets. These model DSCBs have dataset names that are the same as the name of the GDG base, are not cataloged (because you cannot have 2 catalog entries of different types with the same name), and are allocated on the same volume as the catalog containing the GDG base. Originally, this type of model DSCB was required, or a DCB parameter pointing to a general purpose model DSCB had to be specified. For many years a system update was available that removed this requirement; I don't know if this update has been maintained for recent MVS and z/OS releases.
  
  The model DSCB requirement has been removed for SMS managed GDG datasets because it may not be possible to allocate the model DSCB datasets on the appropriate volume, and, in the SMS world all datasets must be cataloged.

Also, since the introduction of DFSMS the need for a model DSCB has been eradicated. Be it that the GDS is SMS managed or not, there is absolutely no need for model DSCBs anymore

Well, unless your storage people have set up the environment incorrectly