Using VSAM Data Sets

This section explains how to use alternate indexes to VSAM data sets.

Alternate Index Files

DFM support for VSAM alternate indexes is provided by a DDM file model called an alternate index file (AIF). A DDM AIF provides access to a VSAM base data set (an ESDS or a KSDS), through a VSAM alternate index path. You can define multiple AIFs over a single VSAM base data set. DFM will create an AIF with RECSZ(4086 32600). The RECSZ parameter on the chosen DATACLAS will be ignored by IDCAMS.

You can choose a DDM AIF name with a maximum of 40 characters.

Base Data Sets

You can use DFM to create an alternate index over an ESDS or KSDS base data set. Before you can build an alternate index, the base data set must meet the following requirements:

DFM requirements:
- If the base was created using DFM, it must be a KSDS.
- If it was created by a local z/OS® user, it can be an ESDS or a KSDS.
VSAM requirements:
- The base data set must contain records.
- The base data set must be nonreusable. For more information, see REUSE Attribute for VSAM Data Sets.

Access Restriction

After you create an alternate index file or files, you will have more than one access path to the same VSAM base data set. At that point, you can access the base data set directly or through an alternate index path. However, to avoid locking conflicts, you are advised to use only one access path at a time.

DFM uses VSAM data definition name (ddname) sharing, VSAM SHAREOPTIONS, and the DFM lock manager to ensure data integrity. For simultaneous use of more than one access path at a time, consult z/OS DFSMS Using Data Sets for additional information about VSAM sharing.