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

time in seconds
6.1.2 Conclusion
180
160
140
120
100
Figure 6-3 SHRLEVEL CHANGE vs. SHRLEVEL REFERENCE w/o FlashCopy
Online CHECK INDEX (SHRLEVEL CHANGE) dramatically cuts down elapsed time. The first
reason is that the parallel processing structure minimizes elapsed time in cross checking the
data and indexes. The second reason is using FlashCopy V2. Compared to online CHECK
INDEX with a non-partitioned index, online CHECK INDEX with a partitioned index could
provide better performance. The most important aspect from the availability point of view is
that using CHECK SHRLEVEL(CHANGE) with FlashCopy 2 the table and the indexes are
available for RW after a few seconds, while with SHRLEVEL(REFERENCE) they are both in
UTRO for most of execution time.
Another study shows the better performance of CHECK INDEX with SHRLEVEL CHANGE in
one task, when compared to CHECK INDEX with SHRLEVEL REFERENCE as concurrent
jobs of each table space partition. It means you should always choose SHRLEVEL CHANGE
for shorter CPU and elapsed time, rather than SHRLEVEL REFERENCE, when FlashCopy
V2 is available.
The two options have a different behavior:
► SHRLEVEL REFERENCE Unload phase unloads keys from the indexes and then the
Check phase compares sorted keys against the data via tablespace scan.
► SHRLEVEL CHANGE Unload phase does a tablespace scan extracting keys and then the
Check phase compares sorted keys against the keys in the index.
6.1.3 Recommendations
80
60
40
20
0
Check Index SHRLEVEL CHANGE versus
SHRLEVEL REFERENCE no FlashCopy
PI NPI
CPU SHRLEVEL(REFERENCE) CPU SHRLEVEL(CHANGE)
Elapsed time SHRLEVEL(REFERENCE) Elapsed time SHRLEVEL(CHANGE)
Use online CHECK INDEX whenever availability is needed. You should use online CHECK
INDEX in conjunction with FlashCopy V2, otherwise concurrent copy is invoked which
increases elapsed time and elongates the time when the data and index are read-only. To
Chapter 6. Utilities 267