Indexed Sequential Access Method (ISAM)

ISAM is an access method that supports sequential and also direct processing of data on CKD disk devices of earlier design.

To process a file, ISAM requires that the file's records have keys of a fixed length. ISAM maintains a two- or three-level index of the highest record keys on the tracks used by a file.

ISAM can handle a file overflow within the limits of available extents.

ISAM does not:
  • Handle records of variable length
  • Handle a file that is not entirely online

For performance reasons, a file accessed by ISAM needs to be reorganized from time to time. This requires that twice the size of the file is kept online or that a tape is used as intermediate storage.

IBM has discontinued further development effort for this access method. Consider using VSE/VSAM instead of ISAM if certain data on disk is to be accessed both sequentially and direct.

The access method is not discussed any further in this publication. The macros in support of this access method are documented in Referencing z/VSE Macros.