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

4.5.1 Performance
The most buffer pools DB2 allows you to page fix are up to 80% of the real storage of the
z/OS LPAR. This limitation is in place to protect other workloads on the LPAR from being
“squeezed” by DB2.
We tested the impact of long term page fixing all of our buffer pools, both in a data sharing
and non-data sharing environment. We used the same IRWW workload for both tests. We
also used an I/O intensive workload, by limiting the storage used by the buffer pools to 400
MB.
Let us first look at non-data sharing. Table 4-9 compares PGFIX(NO) to PGFIX(YES) in a
non-data sharing test.
Table 4-9 Long term page fix - non-data sharing
PGFIX NO YES Delta
(YES/NO)
DBM1
(msec / commit)
Total DB2 CPU time
(msec / commit)
ITR
(msec / commit)
Page fixing DB2 buffer pools has a significant positive impact on performance. We can see
almost a 30% reduction in CPU consumed by the DBM1 address space per transaction. This
is because prefetch requests and deferred write I/O requests stand to benefit the most from
page fixing. DB2 no longer has to page fix a large number pages before initiating each
prefetch and write I/O request. DB2 also does not have to release the pages after the I/O has
completed. DB2 only incurs the cost of page fixing when the buffer pool page is first
referenced in an I/O request. Subsequent I/Os do not incur this cost again.
We can also see a 8% reduction in total CPU time used by DB2 and a gain of 6% in
transaction throughput.
Now let us have a look at data sharing. Table 4-10 compares PGFIX(NO) to PGFIX(YES) in a
data sharing environment with two members. Notice that a long term page fix applies to GBP
read and write. It does not apply to castout work area, since cast work area is not a part of
buffer pool and a long term page fix is a buffer pool option.
Table 4-10 Long term page fix - data sharing
170 DB2 UDB for z/OS Version 8 Performance Topics
0.455 0.322 -29%
2.436 2.238 -8%
587.22 601.42 +6%
PGFIX NO YES Delta
(YES / NO)
DBM1
(msec / commit)
Total DB2 CPU time
(msec / commit)
Group ITR
(commit / sec)
0.560 0.563 0.456 0.457 -19%
3.318 3.306 3.109 3.357 -2%
890.70 897.00 +1%