Storage estimate for caching no data

For planning purposes, you can use a formula to estimate the size of group buffer pools that cache no data.

The following is a formula for estimating storage for group buffer pools that cache no data (GBPCACHE(NO)):

  Dir_entries  = U * VP
  Dir(MB)      = 1.1 * Dir_entries*0.2/1024
  GBP(MB)      = Dir(MB)
  RATIO        = n/a

If the group buffer pool itself is defined with GBPCACHE(NO), the ratio is ignored.

In summary, the variables are:

U: The estimated degree of data sharing.
P: The page size (4 KB, 8 KB, 16 KB, or 32 KB).
VP: The number of data pages 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 very high (U=1).
Member 1 is configured with a virtual pool of 80000 buffers.
Member 2 is configured with a virtual pool of 40000 buffers (VP=120000).

The following calculation is used:

Dir_entries  = 1 * 120000 = 120000
Dir(MB)      = 1.1 * 120000 * 0.2/1024 = 25 MB
GBP(MB)      = 25 MB

The preceding calculation indicates that the group buffer pool should be defined with an INITSIZE of 25 MB. Use the command ALTER GROUPBUFFERPOOL to change the GBPCACHE attribute to NO. If you put GBPCACHE NONE page sets in a GBPCACHE(YES) group buffer pool, the calculation becomes more complicated. You do this because the RATIO is observed and you are probably going to waste a lot of space on unneeded data entries.