2. PLANNING › 2.2 Supported MVS Data Structures › 2.2.3 VSAM Catalog Support

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2.2.3 VSAM Catalog Support


VCC supports both master and user catalogs.  It scans the low
key-range portion of a VSAM catalog to collect and records
the following:

   o  VSAM catalog self-describing information
   o  VSAM data set information
   o  VSAM alternate index information

VSAM Catalog Overview

A VSAM master or user catalog is a VSAM key-sequenced data
set (KSDS) that is divided into two sections:  the low
key-range and the high key-range.  Each control interval
(physical record) in a VSAM catalog is 512 bytes long.  Each
of the logical records in the low key-range portion of a
catalog are one control interval in length.  The low
key-range portion of a catalog contains detailed information
about the following:

   o  VSAM data sets
   o  VSAM alternate indexes
   o  VSAM paths
   o  VSAM data spaces
   o  DASD volumes owned by VSAM
   o  VTOC data set volume pointers
   o  OS/CVOL pointers
   o  Alias associations with OS/CVOLs or VSAM user catalogs

The logical records in the high key-range portion of a
catalog are 47 bytes long and there can be several of these
records in one control interval.  These records are referred
to as "truename" catalog records.  They are comprised of the
following:

   o  A 44-byte data set name of a data set, cluster, user
      catalog, VTOC data set, alternate index, path, alias
      (blank-filled on the right), or volume serial number
      (zero-filled on the right).  This is the key.

   o  A three-byte control interval number of the catalog
      record in the low key-range portion of the catalog that
      describes the object identified above.

A catalog is therefore accessed by doing a keyed read
specifying the data set name of the object being located.
The truename record is returned.  VSAM then uses the control
interval pointer in the truename record to retrieve the
remainder of the information on the object being located.

For more information on VSAM catalog structure, see the IBM
Publication SY26-3826, OS/VS2 MVS Catalog Management Logic.