PR/SM formulas for calculating sizes of group buffer pools

You can use Processor Resource/System Manager (PR/SM) formulas to calculate the sizes of group buffer pools.

You can also calculate group buffer pool sizes by using the coupling facility allocation formulas for cache structures found in 32,13: zSeries Processor Resource/Systems Manager Planning Guide. The following table contains information that is used in those formulas. The size of cache structures in Db2 can vary greatly based on the amount of data for which inter-Db2 read/write interest exists at any given time. You probably need to monitor the use of the group buffer pools and adjust their sizes accordingly.

Table 1. Information for calculating cache structure sizes
Parameter Db2 Value Explanation
MSC 1 Maximum storage class
MCC 1024 Maximum castout class
MDAS Dependent on page size being cached:
  • 1 for 4 KB pages
  • 2 for 8 KB pages
  • 4 for 16 KB pages
  • 8 for 32 KB pages
 Maximum data area size
DAEX 4 Data area element characteristic
AAI 0 Adjunct assignment indicator
R_de Set on ALTER GROUPBUFFERPOOL. Default is 5. See the table below for more information on determining this value. The directory portion of the directory-to-data ratio
R_data Set on ALTER GROUPBUFFERPOOL command. See the table below for more information on determining this value. The data object portion of the target directory-to-data ratio

The following table contains information about the formulas used to determine R_data and R_de. N is the RATIO entered on the ALTER GROUPBUFFERPOOL command. If N has a decimal point, all digits after the first decimal place are ignored. For example, 5.67 is treated as 5.6.

Table 2. Formulas for determining R_data and R_de.
Page Size N has no decimal point N has decimal point
4 KB R_de N N×10
4 KB R_data 1 10
8 KB R_de N N×10
8 KB R_data 2 20
16 KB R_de N N×10
8 KB R_data 4 40
32 KB R_de N N×10
32 KB R_data 8 80