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

service for Unicode conversions, and prior to z/OS 1.4 the conversion service was very slow,
especially on pre-z900 processors.
DB2 V8 brings many more enhancements in DB2’s support for Unicode as well as removing
many of the limitations that were in V7. When you first start V8, you are running in CM in
which the DB2 catalog is still in EBCDIC and you are prevented from exploiting new function.
However, all SQL parsing in V8 is done in Unicode, (that is, UTF-8), even in compatibility
mode. SQL statements that are input in EBCDIC, ASCII or UTF-16 must be converted to
UTF-8. In compatibility mode, all the DB2 object names the SQL statements reference must
again be converted to EBCDIC in order to compare them to DB2 catalog data. In addition, all
Utility control statements must be converted to Unicode before they are parsed.
When you migrate the catalog to new-function mode (NFM), the DB2 catalog is converted to
Unicode, and fallback to V7 is no longer possible. New DB2 V8 subsystems always run in
NFM. SQL statement conversion is unaffected by the switch to NFM, but all the DB2 object
names the SQL statements reference no longer need to be converted since the DB2 object
names are already in the CCSID of the DB2 catalog. DB2 V8 also introduces the capability to
join two tables of different CCSIDs. Needless to say, Unicode conversion is necessary to do
this and the extra CPU time is likely to be substantial.
Now, let us turn to understanding how or when CCSID conversion impacts performance.
Recall that ‘class 1 CPU time’ and ‘class 2 CPU time’ are CPU metrics reported in the DB2
accounting statistics. Class 1 CPU time represents the CPU time consumed between the
creation of a DB2 thread and the termination of the thread. Class 2 CPU time is a subset of
the class 1 CPU time and it represents the CPU time from when the application passed the
SQL statements to the local DB2 system until return. Sometimes, class 2 time is referred to
simply as ‘in DB2 time’. In general, conversions performed by remote clients affect neither
class 1 nor class 2 CPU time, but conversions performed by local Java clients only affect
class 1 CPU time. The DBM1 address space handles all CCSID conversions on behalf of old
mainframe legacy applications, including DB2 utilities. The DIST address space handles all
metadata conversions for remote applications, if any conversion is needed by DB2.
Let us have a look in more detail. Figure 4-12 shows the flow of data for a typical legacy
application accessing a Unicode table, where the application is bound using the default
application encoding scheme, which is EBCDIC.
Local Application DB2 DBM1
INSERT
HV1 37
HV2 37
SELECT
HV1 37
HV2 37
Figure 4-12 CCSID conversion - Local application
176 DB2 UDB for z/OS Version 8 Performance Topics
Class 2 CPU Time
1208 COLC
1200 COLG
1208 COLC
1200 COLG