Monitoring I/O for application program DL/I calls

The IMS Monitor report shows the total number of I/O occurrences and the total time the occurrences took for each UOR during a monitored interval.

The Program I/O report gives these two totals for all PSBs active during the monitored interval and includes the detailed breakdown of the I/O wait time as it was incurred by each PCB used by the program.

The report shows any contention experienced during application program processing. Each type of conflict and the number of times it occurred are recorded for each I/O PCB or database PCB. The report shows the total wait time, the highest wait experienced, and the average time. Subtotals are given for each PCB under a PSB, and for all PCBs under each PSB.

The DDN/FUNC column list the data set by DD name. The MODULE column uses a code to indicate the source of the contention. The types of conflicts and codes are shown as follows. Any codes that appear apply to IMS only.

• Scheduling

  Code

  Conflict

  BLR

  Load/read from ACBLIB

  SMN

  Virtual storage management

• Database access

  Code

  Conflict

  DBH

  OSAM I/O

  DLE

  DL/I functions

  VBH

  VSAM interface

  (Physical segment code)

  Program isolation

The I/O waits for the calls to the I/O PCB are grouped as the first entries for a PSB. For DL/I calls, the data set for which the I/O took place is indicated under the DDN/FUNC heading, and the module code tells you what type of conflict caused the wait.

Contention for the same physical segment in a database causes a wait on behalf of program isolation. This is shown in the DDN/FUNC column, on the PCB line, by the entry PIdmb, where dmb is the DMB of the physical data set. The MODULE column identifies the segment type using the physical segment code assigned by DBD generation.

When an application is accessing a database using VSAM as the access method, DL/I calls do not generally result in an I/O wait. A MODULE column entry of VBH indicates that interface to VSAM occurred and there was an I/O wait.

A seemingly unrelated entry can occur under the DDN/FUNC column for a database PCB. An example is a retrieval call to a database (DB-A) that causes a buffer to be purged in order to make room for that retrieved data. If the buffer contents included data belonging to another database (DB-B), the I/O entry in the report shows the DD name for DB-B as being in conflict for PCB access to DB-A.