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

achieved by simply moving to DB2 V8. DB2 V8 is able to exploit the 64-bit interface into the
z/OS Conversion Services, which is much more efficient than the 31-bit interface used by
DB2 V7. Next, we see a greater improvement by moving to the z990 hardware, which brings a
number of specific hardware instructions to support Unicode conversion. Next, we introduce
optimization in DB2 V8 called “minor conversion”. FInally, we introduce further optimization in
DB2 V8 which in certain cases allows the system to avoid conversion.
Now, we explore each of these enhancements in more detail.
DB2 V8 major and minor conversion
Conversion of SQL statements and DB2 object names generally involves English
alphanumeric characters. For many years the DB2 Catalog has been EBCIDC. To improve
the performance of DB2, V8 developed a faster way to convert such characters without calling
the z/OS Conversion Services. This faster way is called minor conversion. Therefore, the term
major conversion refers to those conversions performed by the z/OS Conversion Services.
DB2 maintains a translate table associated with the EBCDIC SBCS/mixed CCSIDs that are
specified during DB2 installation. The same translate table is also used for access to the DB2
catalog and user data. Minor conversion is supported for all European EBCDIC character
sets, and some Asian EBCDIC character sets. Minor conversion does not apply to UTF-16
(GRAPHIC) or ASCII; it may only apply to conversions between EBCDIC and UTF-8.
How does minor conversion work?
The zSeries instruction set has always included a TR instruction that translates one byte for
one byte, based on a 256-byte translation table. A TRT instruction has also existed which
could be used to test a string for certain one byte characters. As long as a single byte English
alphanumeric character can be represented in UTF-8 by a single byte, a TR instruction can
be used to convert the byte to UTF-8. DB2 V8 has built-in translate tables for most common
single byte CCSIDs and if the TRT instruction is "successful", DB2 can translate the string
using a TR instruction. The presence of shift-in/shift-out characters in a mixed string always
causes the TRT instruction to fail. If the TRT fails, then DB2 must invoke the z/OS Conversion
Services.
The performance of minor conversion is much better than the performance of major
conversion because minor conversion avoids calling the z/OS Conversion Services. Hence,
minor conversion is one of the significant advantages of DB2 V8 compared to DB2 V7, and
represents an advantage of UTF-8 over UTF-16. Even when DB2 V8 needs to use major
conversion, V8 is still much faster than V7, which we will see later.
Note that DB2 V8 still uses the catalog table SYSIBM.SYSSTRINGS for conversions that do
not involve Unicode.
DB2 V8 introduces a further optimization for ASCII conversion to UTF-8. DB2 uses the TRT
instruction to ensure all characters are less than 128, and because the first 128 code points
are the same between ASCII and UTF-8, no conversion is necessary. DB2 can skip
conversion altogether.
Studies using strings containing only English characters show that minor conversion in V8
can be more than four times faster than major conversion in V8. Compared to conversion in
V7, minor conversion can be more than six times faster. Keep in mind that minor conversion is
only successful for single byte characters.
z/OS Conversion Services
The z/OS Conversion Services contain both a 31-bit and a 64-bit service. DB2 V7 uses the
31-bit service, while DB2 V8 most often uses the 64-bit service.
Chapter 4. DB2 subsystem performance 185