Chapter 8. DB2 139
8.2.2 Page sizes
Physical objects on disks, such as tables and indexes, are stored in pages. DB2 supports
4 KB, 8 KB, 16 KB, and 32 KB page sizes. During the processing of such objects, they are
brought into DB2 buffer pools in main memory. The default AIX page size is 4 KB, but other
page sizes are available. To achieve increased performance on Power Systems, DB2 10.1 by
default uses 64 KB, which is a medium page size.
Large page size
For some workloads, particularly ones that require intensive memory access, there are
performance benefits in using large page size support on AIX. However, certain drawbacks
must be considered. When large pages support is enabled through DB2, all the memory that
is set for large pages is pinned. It is possible that too much memory is allocated for large
pages and not enough for 4 KB pages, which can result in heavy paging activities.
Furthermore, enabling large pages prevents the STMM from automatically tuning overall
database memory consumption. Consider using this variable only for well-defined workloads
that have a relatively static database memory requirement.
The POWER7 large page size support can be enabled by setting the DB2 registry variable
DB2_LARGE_PAGE_MEM.
Here are the steps to enable large page support in DB2 database system on AIX
operating systems:
1
1. Configure AIX server for large pages support by running vmo:
vmo -r -o lgpg_size=LargePageSize -o lgpg_regions=LargePages
LargePageSize is the size in bytes of the hardware-supported large pages, and
LargePages specifies the number of large pages to reserve.
2. Run bosboot to pick up the changes (made by running vmo) for the next system boot.
3. After reboot, run vmo to enable memory pinning:
vmo -o v_pinshm=1
4. Set the DB2_LARGE_PAGE_MEM registry variable by running db2set, then start the DB2
database manager by running db2start.:
– db2set DB2_LARGE_PAGE_MEM=DB
– db2start
8.2.3 Decimal arithmetics
DB2 uses the hardware DFP unit in IBM POWER6 and POWER7 processors in its
implementation of decimal-encoded formats and arithmetics. One example of a data type that
uses this hardware support is the DECFLOAT data type that is introduced in DB2 9.5. This
decimal-floating point data type supports business applications that require exact decimal
values, with precision of 16 or 34 digits. When the DECFLOAT data type is used for a DB2
database that is on a POWER6 or POWER7 processor, the native hardware support for
decimal arithmetic is used. In comparison to other platforms, where such business operations
can be achieved only through software emulation, applications that run on POWER6 or
POWER7 can use the hardware support to gain performance improvements.
1
DB2 Version 10.1 for Linux, UNIX, and Windows, Enabling large page support (AIX), available at:
http://pic.dhe.ibm.com/infocenter/db2luw/v10r1/index.jsp?topic=%2Fcom.ibm.db2.luw.admin.dbobj.doc%2F
doc%2Ft0010405.html
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