When defining pre-existing native VSAM files to CA-IDMS, it is necessary to define page ranges. Such files are first and foremost native VSAM files and have no concept of IDMS page ranges. This article describes what those page ranges are used for.
CA IDMS, all supported releases.
The page ranges defined on a native VSAM area are used for generating a pseudo database-key which is then used for locking purposes within IDMS. For ESDS and RRDS files, the page range should be defined according to the number of records in the file. For KSDS files, the value of the record's primary key is randomly hashed to generate a database-key within the specified page range. The greater the page range, the better granularity of locking will be achieved. The smaller the page range, the more is the chance that multiple records will hash to the same value, and so a lock placed on one of those records will lock them all.
AREA Statements (scroll down to "Page Range for RRDS Native VSAM Areas" etc.).