30 DB2 UDB ESE V8 Performance Guide for High Performance OLTP and BI
2.2.4 Memory model
Different applications use memory in different ways. For example, some
applications may use the operating system cache for file handling, while the
database manager uses it own buffer pool for data caching. Therefore, in order to
achieve good performance, it is important to balance overall memory usage on
the system to minimize paging, as described in 5.2.2, “Memory considerations”
on page 335 and 5.3.2, “Memory considerations” on page 383. This requires a
good understanding of how DB2 organizes memory.
Memory allocation is based on database manager and database configuration
parameters. These parameters may specify hard or soft upper limits, lower
bound values, or let DB2 automatically allocate memory resources as required
via the AUTOMATIC setting. Depending upon the area of memory involved, DB2
allocates and deallocates memory at different times; some of the values of these
parameters can be changed online to take immediate effect.
Figure 2-6 on page 31 shows the different portions of memory that the database
manager allocates for various uses.
db2logts This process is used for collecting historical information about which
logs are active when a table space is modified. This information is
recorded in the DB2TSCHG.HIS file in the database directory. It is
used to speed up table space roll forward recovery.