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

4.10.2 Conclusion
4.10.3 Striping
All I/O bound executions benefit with using larger VSAM CI sizes, including COPY and
REORG. This enhancement can also potentially reduce elapsed time for table space scans.
There is little to be gained in performance by going above 16 KB CI size.
Now, for a few brief words on VSAM striping. In DB2 V7, VSAM striping was only
recommended for DB2 log data sets and DB2 work file data sets. Although you could also use
VSAM striping for application data with 4 KB pages, it was not generally recommended or
widely used, because it was not extensively researched or tested. However, some customers
have used VSAM striping rather successfully in V7 on DB2 indexes, to dramatically increase
the performance of REBUILD INDEX. Once you have saturated your channels, VSAM striping
does not really help.
I/O striping is beneficial for those cases where parallel partition I/O is not possible, for
example, segmented tablespaces, work files, non-partitioning indexes, and so on.
4.10.4 Recommendations
If you already have large pages, migrate them to large CIs. The default CI size is equal to the
page size. If you have large table space scans, larger CIs can also help.
4.11 IBM System z9 and MIDAWs
MIDAW stands for Modified Indirect Data Address Word facility, and as the name says, it is a
modification to the standard IDAW channel programming technique that has existed since the
S/360 days. MIDAWs adds a new method of gathering data into and scattering data from
discontiguous storage locations during I/O operations. This improves the performance of
FICON channels.
MIDAWs require the IBM System z9 server and z/OS Release 1.7 or APAR OA10984 with
z/OS Release 1.6, see Figure 4-21.
202 DB2 UDB for z/OS Version 8 Performance Topics