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

make DB2 take the maximum parallelism in sort tasks it is recommended to estimate the sort
work data set size correctly.
Note: APAR PQ92749 and PQ96956 provide the CHECK INDEX availability
enhancement.
6.2 LOAD and REORG
SORTKEYS for LOAD and REORG, and SORTDATA for REORG are keywords which
generally contribute to better performance. In V7 SORTKEYS and SORTDATA were not the
default, so you had to specify explicitly these parameters to activate the methods providing
better performance. In V7 if SORTKEYS is not specified, and if data is 100% clustered and
there is only one index on the table, LOAD utilities skip the SORT phase. If you want to exploit
the functions, you need to change the utility jobs to add these keywords. In V8, SORTKEYS in
the LOAD utility is the default and SORTDATA and SORKEYS are the default in the REORG
utility. The SORT phase is enforced in LOAD and REORG in V8.
SORTKEYS cuts down the elapsed time of the index key sort by reducing I/O to the work file,
since using SORTKEYS index keys are passed in memory rather than written to the work file.
In addition, SORTKEYS provides a method of parallel index build. If there is more than one
index on your table space or partition, it reduces the elapsed time of LOAD jobs.
SORTDATA in the REORG utility specifies that the data is to be unloaded by a table space
scan, and sorted in clustering order. This allows a consistent execution time for REORG and
good performance whether the data is disorganized or not.
6.2.1 LOAD with default performance measurement
The following performance measurement has been done to observe LOAD performance with
the default option of SORTKEYS. The purpose of the measurement is to assess the
performance benefit of the SORTKEYS method provided as default. In this measurement we
run a comparison of LOAD utilities in V7 and V8 over table spaces with 1 index, 2 indexes,
and 6 indexes. The following is the description of our test environment.
► 1 partitioned table with 10 partitions
► 1,000,000 rows in one partition (total 10 million rows) and 118 byte row size
► 26 columns of CHAR, INTEGER, and DECIMAL data type
Figure 6-4 summarizes the results.
LOAD executed against the table space with just one index shows up to 11% CPU
degradation and 7% elapsed time degradation in V8 compared to V7. When more than 1
index exists, V8 performs better in terms of CPU and elapsed time. The dramatic elapsed
time reduction in V8 with 6 indexes comes from parallel processing. One thing to be
considered is that more work file space is needed to accommodate parallel processing.
268 DB2 UDB for z/OS Version 8 Performance Topics