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

Because you can significantly increase the size of the buffer pools; theoretically, you can
dramatically reduce the number of synchronous I/Os and therefore reduce the elapsed time
for most transactions. This can significantly improve batch performance, especially for those
workloads where the number of synchronous I/Os is high. As a result, DB2 V8 can provide a
good real memory versus I/O trade-off. However, you need to monitor real storage usage.
In general, we have experienced an increase of up to 10-20% in real storage going to DB2 V8
when keeping constant all the possible real storage requirements. If you are close to fully
exploiting your real storage, it is safer to verify the exact usage with RMF reports and
review the virtual storage definitions, including the new default values, in order to contain the
demand on real storage.
2.5 Compatibility mode
Once you migrate to CM, presumably there is no new V8 function available. Well, actually
there is no new function that would preclude a fallback. But new function is introduced. There
are several changes you have absolutely no control over that automatically occur behind the
scenes. The list below, compiled during this project, gives you an idea of what is available, but
by no means can be considered precise or constant, because it is subject to change with
maintenance:
► 64-bit addressing for DBM1 and IRLM address spaces
► Increased number of deferred write, castout and global buffer pool write engines
► EDM pool changes
► Fast log apply default
► Multi-row operations with DRDA
► IRLM locks in private storage
► Unicode parser
► Most of the optimizer enhancements
► Lock avoidance in singleton SELECT with CURRENT DATA YES
► 180 CI limit removal for list prefetch and castout read
► Ability to page fix buffer pools for long term
► SMF 89 improvement
► Data sharing IMMEDIATE WRITE default change
► All the enhancements provided by utilities, except for BACKUP and RECOVER SYSTEM
2.5.1 64-bit addressing
These situations and their performance implications are discussed in this section.
When you migrate to DB2 V8 in CM, the DBM1 and IRLM address spaces immediately start
to use 64-bit addressing. You no longer have the option to allow either address space to run
using 31-bit addressing. Applications on the other hand happily run as before; there is
absolutely no need to make changes to the application, unless of course, you want to take
advantage of new features.
Figure 2-1 shows the virtual storage layout for the different address spaces. Notice the IRLM
and DBM1 address spaces use 64-bit virtual addressing, where DDF, MSTR, SPAS and the
user-defined application program remain as before.
Also, it is important to realize that the storage for 64-bit is not drawn to scale; the DBM1 and
IRLM address space have a huge amount of virtual storage, 8 billion times the other address
spaces.
18 DB2 UDB for z/OS Version 8 Performance Topics