High number of times access path recalculated due to change in SYSPLEX configuration.

Products

• SYSVIEW Performance Management Option for DB2 for z/OS

Issue/Introduction

What conditions will cause the following message to appear in
Sysview Performance Management Option for DB2 (IDB2) exceptions?:

HIGH NUMBER OF TIMES ACCESS PATH RECALCULATED DUE TO CHANGE IN SYSPLEX CONFIGURATION

Resolution

This exception is raised when the number of active members in the data sharing group,
or the processor configuration itself, has changed since the bind was done. 

The following are details on the cause of the Sysview Performance Management
Option for Db2 exception: 

REFORM PARAL-CFG 
The total number of parallel groups where Db2 reformulated the 
parallel portion of the access path because of a change in 
the number of active members, or because of a change of processor
models on which they run, from bind time to run time. This counter
is incremented only on the parallelism coordinator at run time. 


Field Name:
QXREPOP1 

*** 

Bind time activity 
As with parallelism in general, Db2 tries to determine a parallel degree
and a parallel mode (I/O, CP, Sysplex) at bind time. Db2 looks at processor 
speeds and the number of central processors on each member when it
determines the degree of parallelism. 

Check PLAN_TABLE data output to view the access path that Db2 plans 
to use for a particular query. An 'X' value in the PARALLELISM_MODE 
column indicates that Sysplex query parallelism is chosen for the 
explainable statement. If only one member of the data sharing group 
is active at bind time, Db2 can still choose Sysplex query parallelism. 
This lets any assistants that are active at run time help with processing 
the query. 

Db2 can use a different parallel degree at run time if any of the 
following events occur between bind time and run time: 
The number of active members changes, or the processor configuration changes. 
This event is recorded on the parallelism coordinator at run time in the
QXREPOP1 field of the statistics and accounting traces.
(See the REFORM PAR-CONF field in the partial accounting trace.) 
*** 

If Db2 subsystem performance is not impacted. the specific IDB2 exception that is 
reporting on this may be stopped.