Monitoring and tuning group buffer pool checkpoint intervals

You can monitor your group buffer pool checkpoint intervals and adjust the intervals if necessary.

Procedure

To monitor and tune group buffer pool checkpoint intervals:

  1. Periodically monitor your group buffer pool checkpoint by using any of the following options:
    • Issue the DISPLAY GROUPBUFFERPOOL command.
    • Use IFCID 0261, which gives summary statistics for each group buffer pool checkpoint. You can use this record to estimate the processor cost and to monitor the coupling facility interactions for each group buffer pool checkpoint.
    • Use IFCID 0263, which gives summary statistics for the castouts. You can use this record to monitor the castout activity that is caused by each group buffer pool checkpoint (or triggered for any other reason).
  2. Tune the group buffer pool checkpoint as necessary.
    • If the resource consumption of the group buffer pool checkpoint is higher than you prefer:
      • Apply the proper maintenance and allocate the group buffer pool in a coupling facility at CFLEVEL=5 to take advantage of the checkpoint performance enhancement.
      • Increase the checkpoint interval to have the checkpoint occur less frequently.
    • If the checkpoint is not moving the recovery LRSN forward fast enough, decrease the checkpoint interval. You can determine the LRSN by periodically issuing the DISPLAY GROUPBUFFERPOOL command.
    • If the recovery LRSN for group buffer pool checkpoint is not advancing as fast as you want, determine if there have been any disk or coupling facility connectivity problems that are impairing the ability of Db2 to cast out.

Examples

In the following example, the DISPLAY GROUPBUFFERPOOL command shows which member is the structure owner and also shows the group buffer pool checkpoint recovery LRSN.

Figure 1. Partial DISPLAY GROUPBUFFERPOOL output showing which member owns the structure and the group buffer pool checkpoint recovery LRSN
DSNB798I -DB1A LAST GROUP BUFFER POOL CHECKPOINT 17:23:21 OCT  4, 2002
               GBP CHECKPOINT RECOVERY LRSN              = ACD74C01EE30
               STRUCTURE OWNER                           = DB1A

In the following example, the DISPLAY GROUPBUFFERPOOL command with MDETAIL option contains the number of checkpoints that occurred for this group buffer pool. The statistics trace also includes this information.

Figure 2. DISPLAY GROUPBUFFERPOOL MDETAIL report
DSNB778I -DB1A CASTOUT THRESHOLDS DETECTED
               FOR CLASSES                               = 3
               FOR GROUP BUFFER POOL                     = 1
             GBP CHECKPOINTS TRIGGERED                   = 1
             PARTICIPATION IN REBUILD                    = 0
DSNB796I -DB1A CASTOUTS
               PAGES CASTOUT                             = 18
               UNLOCK CASTOUT                            = 3
               READ CASTOUT CLASS                        = 5
               READ CASTOUT STATISTICS                   = 6
               READ DIRECTORY INFO                       = 0

If you are experiencing surges of coupling facility use, it could be related to group buffer pool checkpointing. Examine the number of read directory info requests. If there are many requests per checkpoint, you can probably benefit from applying the proper maintenance to use the group buffer pool checkpoint performance enhancement.