DCB is 'Data Control Block' specifies attributes of the records in the data-set. DCB parameters will be present only when creating a data set. There may be a few exceptions, but these will be determined by operating system requirements. You may use the DCB refer back when creating more than one data set with the same DCB characteristics.
A DSCB is a Data Set Control Block which represents an entry in the VTOC. There are several different types and generally what people see when they list a VTOC is the Format 1 DSCB. There are now 9 formats used, which include the supported Extended Addressability Volume [EAV] attributes.
The role of a model DSCB was specific only to generation data sets and represented a VTOC entry [which was allocated no space] to represent the basic attributes of the generation data set being created. This could be represented by any VTOC entry and often employed the use of the data set name in the DCB parameter [DCB=(HLQ.MDSCB)]
Model DSCB's do NOT exist as data sets. Catalog entries are simply an entry where a name is associated with a device type and volume serial number. If that name is not on the stated volume, then a S213 abend will occur, so catalogs are not any indication of existence.
Similarly a Format 1 DSCB that describes no space is simply a VTOC record entry. It also is inaccessible and only provides data set attribute information. As a result, the data set name being referenced, describing a model DSCB, does NOT exist.
To know more on DSCB read here: Data Set Control Block (DSCB) Types.
Long ago, before you can create the files that attach to a GDG base, you need to create a model data-set that contains the DCB information for the files. This model data just needs to be a VTOC entry, it does not need to actually use up disk space. It is possible to create a different model data-set for every possible DCB combination, and some sites actually do that, but this needs a lot of effort to maintain. Many sites use a single model data-set, and then override the DCB information to build the correct type of file. To create a model DCB, use a dummy IEFBR14 job like this:
Code: Select all
//STEP2 EXEC PGM=IEFBR14
//MODEL1 DD DSN=THIS.IS.A.GDG.MODEL,
The SPACE=(TRK,0) parameter means that this file will just be created as a VTOC entry, it will not use up disk space.
Said that, these DSCBs are no longer required if DFsms is installed and tailored correctly and most of the shops are "tailored correctly these days"!