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DFSDVSM0

DFSDVSM0 is both the DL/I VSAM Buffer Handler and the IMS VSAM Interface Module. It manages the VSAM buffer pools by issuing requests to VSAM.

DFSDVSM0 is invoked by the DL/I buffer handler router DFSDVBH0. It processes VSAM Key Sequenced and Entry Sequenced (KSDS and ESDS) datasets. Some of its functions include:

  • locate a record

  • search the VSAM buffer pool for a record within a range

  • retrieve records for Online Image Copy

  • mark a record as altered

  • insert a KSDS record

  • create a new ESDS record

  • write a record

  • write all records altered by a dependent region

  • get/free space in the VSAM buffer pool

  • retrieve buffer pool statistics

  • VERIFY a VSAM cluster

  • perform VSAM background write

Hints

The amount of CPU that DFSDVSM0 uses depends on the amount of VSAM activity in the job. If the amount of CPU is significant (greater than 4%), then check the Data Set Characteristics and I/O Facility Usage reports and implement standard IMS database performance improvements.

The VSAM buffer pool statistics keep counts of many of the functions performed by this module. For IMS message processing regions, use the IMS DCMONITOR or IMS log reports. For BMP and batch jobs, you can add a //DFSSTAT DD card to the JCL, and IMS will write the VSAM buffer pool statistics at the end of the job.

Reference Sources

More information is available in the IMS DBSOURCE Library Data Set. See your IMS systems programmer.