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DSNVSR

Module DSNVSR controls the Suspend/Resume/Cancel Synchronization of DB2 tasks. It is part of Agent services management routines that operate on behalf of Allied Agent (user) address spaces. These services provide for creation/termination, task switching, synchronization, and latch management.

Hints

The following are among the conditions that will result in wait time attribution to DSNVSR:

  • wait for Db2 objects locked in an incompatible mode by other users

  • wait for synchronous I/O

  • service task wait such as tablespace and indexspace opens

Wait for synchronous I/O can be reduced by improving the buffer pool hit ratio. The attribution of CPU time to DSNB5RDP (GET PAGE REQUESTS) indicates that get page requests (DSNB1GET) are not being satisfied by pages in the buffer pools. If poor buffer hit ratios are causing I/O wait, use EXPLAIN to examine the access path of the SQL statement in question. If EXPLAIN shows an acceptable access path, check the actual clustering ratio of any index used to evaluate its impact on buffer pool hit ratios.

Encouraging asynchronous prefetch should reduce I/O wait time but may be inappropriate for online applications.

If the statement's access path is good, possibly including prefetch, and the index used promotes a good hit ratio, then examine the buffer pool statistics to determine if your buffer pools are handling the work load efficiently.

Service Task Wait for file opens can be minimized by avoiding the use of CLOSE YES, using SEGMENTED tablespaces, and taking advantage of DEFERREDCLOSE PROCESSING by setting DSNMAX greater than the sum of the number of CLOSE NO datasets and the maximum number of CLOSE YES datasets that are opened and in use at any time.

You can detail high wait in DSNVSR by turning on Class 3 Accounting. The DB2 Performance Monitor report will show types of suspension activity to the column SUSPENSIONS/SYSTEM EVENTS with various events, such as LOCK/LATCH SUSPENSIONS, SYNC.I/O SUSPENSIONS, etc. You can analyze these events further. For example, you can detail LOCK/LATCH SUSPENSIONS on the Lock Suspensions Summary, Lock Contention Summary, and Lock Contention Trace reports. You can examine buffer pool hit ratios on the Accounting Summary report or analyze the Buffer Pool Statistics report. See the Strobe CICS Performance Supplement.

Reference Sources

IBM Db2 for OS/390, Messages and Codes, GC26-8979

IBM Db2 for OS/390, Application Programming and SQL Guide, SC26-8958

IBM Db2 for OS/390, Administration Guide, SC26-8957