Storage estimate for caching LOB space maps (GBPCACHE SYSTEM)

For planning purposes, you can use a formula to estimate storage for group buffer pools that cache LOB space map data.

The following is a formula for estimating storage for group buffer pools that cache LOB space map data: 
Data_entries = (U * D / 10) * R
Data(MB)     = Data_entries * P / 1024
Dir_entries  = Data_entries + (U * VP)
Dir(MB)      = 1.1 * Dir_entries * 0.2 / 1024
GBP(MB)      = Data(MB) + Dir(MB)
RATIO        = MIN(Dir_entries / Data_entries, 255)
In summary, the variables are:
U
The estimated degree of data sharing.
D
The number of data pages that are written to disk per second for all members during peak activity. Do not use the number of pages that are written to the group buffer pool; D must be a count of distinct pages. To determine this value, use the field QBSTPWS from IFCID 0002 (the PAGES WRITTEN field of the buffer pool section of the IBM® OMEGAMON® for Db2 Performance Expert on z/OS® Statistics report).
10
An estimate of the LOB system pages that are written for every LOB data page.
P
The page size (4 KB, 8 KB, 16 KB, or 32 KB).
R
The average page residency time in the group buffer pool in seconds.
VP
The number of data pages that are defined for the virtual pool (the sum across all the members).

Example

Assume that you have a two-member data sharing group for which you have determined the following information:

  • The degree of data sharing is moderate (U=.7).
  • 10 disk writes per second occur across both members during peak activity (D=10).
  • The space map page is resident in the group buffer pool page for 120 seconds (R=120).
  • The page size is 32 KB (P=32).
  • Member 1 is configured with a virtual pool of 20000 buffers.
  • Member 2 is configured with a virtual pool of 10000 buffers (VP=30000).

The following calculation is used:

Data_entries = ((.7 * 10)/ 10) * 120 = 84
Data(MB)     = 84 * 32 / 1024 = 2.6 MB
Dir_entries  = 84 + (.7 * 30000) = 21084
Dir(MB)      = 1.1 * 21084 * 0.2 / 1024 = 4.5 MB
GBP(MB)      = 2.6 MB + 4.5 MB = 7.1 MB
RATIO        = MIN (21084 / 84, 255)  = 251

The preceding calculation indicates that the group buffer pool should be defined with an INITSIZE of 7.1 MB. The ratio is less than the maximum value, so use the command ALTER GROUPBUFFERPOOL to change the ratio to 251.