PARTITIONED DATASETS - IBM Mainframe

A Partitioned Data Set(PDS) is an IBM OS/390 (MVS) file that contains one or more sequential files called members. Members can be used to store various types of information:

  • General alphanumeric data, such as the contents of database records and fields.
  • Text files, such as paragraphs or chapters in a novel.
  • Source files, which are uncompiled job-specific instructions written in a programming language like Fortran, SAS, C, or COBOL.
  • Object decks, which are compiled programs, not yet link-edited.
  • Macros, a series of computer instructions invoked during the translation of a single coded word.
  • Executable programs, stored as machine language, produced by compiling and link-editing a source file. An example would be a subroutine program. A PDS whose members are independently executable is identified in this document as a load module library. The synonyms program library and load library are also used here.
  • Batch job instructions, using Job Control Language (JCL).
  • TSO CLISTs and REXX execs.

Partitioned data sets are versatile.Information in one PDS member can be accessed quickly,directly, without disturbing adjacent members. Members are located via a directory, where each has a unique entry. At the same time, PDS format allows processing of sequential members in batch operations.

Created PDSs should be compact to save on storage costs and conserve disk space, yet flexible enough to permit growth in the data set. For example, if you don't allocate enough directory blocks when you create a PDS you may have to subsequently create another, larger PDS and transfer all your data to it. Conversely, allocating more directory blocks than will ever be used is wasteful and can result in unnecessary disk storage charges.

Physical Representation of a Physical Sequential Data Set

Physical Representation of a Physical Sequential Data Set

You can use TSO/ISPF option 3.2 to allocate a PDS. During the allocation time, you will have to specify how big you want the directory to be by specifying the size of the directory block. You can't change the size of the directory after creation. So if you are planning to have a large number of members in the PDS you should allocate more space to the directory. The following figure shows the physical representation of a Partitioned dataset.

A PDS can exist only in on disk. You can copy its contents to tape, but you can't directly access the data on tape. And a PDS can't straddle multiple disks, it must be contained on one disk. But a single disk can hold multiple PDSs. To code the member names in the JCL you use the DSN parameter. For example, to refer to a member named TEST in my personal library, I can code: DSN=SEGAWL.AWL.PERSNL(TEST)

Physical Representation of Partitioned Data Set

Physical Representation of Partitioned Data Set


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