DB2 UDB for z/OS Version 8 Performance Topics - IBM Redbooks

Table 8-12 DB2 I/O CI Limit Removal - Non-data sharing
Activity With I/O CI Limit I/O CI Limit
Removed
List Prefetch Requests 57,889 58,165
List Prefetch Reads 317,000 56,122
List Prefetch Pages Read /
List Prefetch Reads
The values listed verify that the limit on CIs per DB2 I/O has been removed. The number of
List Prefetch Reads has reduced dramatically, with a corresponding increase in the number of
pages read per List Prefetch request.
Table 8-13 shows extracts from DB2 PE statistics reports for a data sharing environment with
two members running the same IRWW workload.
Table 8-13 DB2 I/O CI limit removal - Data sharing
You can see the positive impact of removing the DB2 CI I/O limits. The number of List
Prefetch Reads has reduced, with a corresponding increase in the number of pages read per
List Prefetch request.
Note also that the number of pages written per write I/O has also increased significantly. This
is unique to data sharing and shows the extra benefits of removing this limitation in a data
sharing environment. The DB2 CI I/O limits is also removed for castout processing in data
sharing environments. The number of pages written per write I/O increased significantly
because more pages can now be written in castout. We would also see a corresponding
decrease in Unlock Castout requests as reported in the Group Buffer pool Activity section of a
DB2 PE Statistics report.
Now we have a look at the impact on CPU. Table 8-14 shows CPU times from DB2 PE
statistics reports for a non-data sharing environment, running the IRWW workload.
Table 8-14 DB2 I/O CI limit removal - Non-data sharing CPU
334 DB2 UDB for z/OS Version 8 Performance Topics
4.76 26.85
Pages Written per Write I/O 1.84 1.83
Activity With I/O CI Limit I/O CI Limit Removed
List Prefetch Requests 42,876 42,783 43,186 43,187
List Prefetch Reads 209,600 208,200 40,894 40,711
List Prefetch Pages Read /
List Prefetch Reads
3.58 3.56 18.44 18.39
Pages Written per Write I/O 6.20 4.37 27.11 26.49
DBM1
(msec/commit)
Total DB2 CPU time
(msec/commit)
ITR
(commits/sec)
With I/O CI LImit I/O CI Limit
Removed
0.342 0.322 -6%
2.281 2.238 -2%
592.47 601.42 +2%
Delta
(Removed / With)