DB2 UDB ESE V8 non-DPF Performance Guide for High Performance OLTP and BI

Chapter 5. Operating system considerations 335

If too many are configured, the threads will not be used; if too few are

configured, performance degrades. While there are several rules of thumb for

determining the correct numbers, we recommend the following:

–Set maxAIOservers to the maximum of:

• Minimum of 10 times the number of database disks, or 10 times the

number of CPUs in the system

• At least one per page cleaner and prefetcher (per instance)

– A good value for minAIOservers is maxAIOservers/2.

The following command displays the current settings of these variables:

lsattr -El aio0

DB2-specific recommendations

We recommend the following DB2-specific best practices:

1. DB2 in 64-bit mode enables DB2 to use larger heaps, but the DB2 processes

such as db2agent also use more memory. This additional memory

(approximately 50 KBytes per process) needs to be factored into memory

usage computations. However, in most situations, heap usage outweighs

process memory usage and the overhead of 64-bit process memory usage is

not a consideration.

2. If you run a DB2 32-bit application on a 64-bit database, you will need to

rebind the application and set UNIX environment variables to point to the

32-bit libraries, for example:

LIBPATH=<db2instance home>/sqllib/lib32

export LIBPATH

3. Multiple versions of DB2 ESE can be installed on the same system. With DB2

V8.1 Fixpak 2, multiple fixpaks of the same DB2 version can also coexist on

the same system.

5.2.2 Memory considerations

In this section, we provide the following:

1. A review of the AIX virtual memory architecture

2. General performance recommendations

3. DB2-specific recommendations

AIX virtual memory architecture review

Virtual memory provides a mechanism for the system to assign more memory to

processes than physically exists on the system. It does this by moving (or

336 DB2 UDB ESE V8 Performance Guide for High Performance OLTP and BI

paging) temporarily unused parts of memory (pages) to disk, and restoring them

when required.

Figure 5-1 shows a simplified model of AIX memory architecture.

Figure 5-1 Simplified view of 32-bit AIX memory architecture

The main elements are real or physical memory, virtual memory addressable

space, segment registers, page tables and page files.

򐂰 All systems have a certain amount of real memory; the maximum varies by

processor model.

򐂰 Process virtual memory, on the other hand, is limited by the architecture: for

32-bit processes, the limit is 4 GB, while for 64-bit processes, the limit is

approximately 2

36

GB.