When existing directory entries are being reclaimed to
handle new work, cross-invalidation must occur for all of the members
that have the particular data pages in their buffer pools, even when
the data has not actually changed.
For example, in the following figure, F indicates
that there have been 18,281 directory reclaims. Figure 1. Example
output of group detail statistics
DSNB783I -DB1A CUMULATIVE GROUP DETAIL STATISTICS SINCE 15:35:23 May 17, 2002
DSNB784I -DB1A GROUP DETAIL STATISTICS
READS
DATA RETURNED A = 3845
DSNB785I -DB1A DATA NOT RETURNED
DIRECTORY ENTRY EXISTED B = 27
DIRECTORY ENTRY CREATED C = 28336
DIRECTORY ENTRY NOT CREATED D = 332, 0
DSNB786I -DB1A WRITES
CHANGED PAGES = 20909
CLEAN PAGES = 0
FAILED DUE TO LACK OF STORAGE E = 8
CHANGED PAGES SNAPSHOT VALUE = 974
DSNB787I -DB1A RECLAIMS
FOR DIRECTORY ENTRIES F = 18281
FOR DATA ENTRIES = 47
CASTOUTS = 16073
DSNB788I -DB1A CROSS INVALIDATIONS
DUE TO DIRECTORY RECLAIMS G = 4489
DUE TO WRITES = 3624
EXPLICIT = 0
DSNB762I -DB1A DUPLEXING STATISTICS FOR GBP0-SEC
WRITES
CHANGED PAGES = 20909
FAILED DUE TO LACK OF STORAGE = 8
CHANGED PAGES SNAPSHOT VALUE = 974
DSNB790I -DB1A DISPLAY FOR GROUP BUFFER POOL GBP0 IS COMPLETE
DSN9022I -DB1A DSNB1CMD 'DISPLAY GROUPBUFFERPOOL' NORMAL COMPLETION
G
indicates that, because of those reclaims, 4489 cross-invalidations occurred. The pages in those
members' buffer pools need to be refreshed when next needed, probably from disk, which can degrade
system performance.
If there is a high value in G , check
the group buffer pool hit percentage to see if the lack of directory
entries might be causing an excessive number of reads from disk.
You can also check the SYNCHRONOUS READS DUE TO BUFFER INVALIDATION counters
shown in the member detail report.
You can increase the number of directory entries in the group buffer
pool in one of two ways:
Increase the total size of the group buffer pool.
Use the ALTER GROUPBUFFERPOOL command to adjust the ratio in favor
of directory entries.
