Getting Started with DB2 Express-C

66 Getting Started with DB2 Express-C
Figure 4.8 –Table spaces created by default when a database is created
Table space SYSCATSPACE contains the System Catalog tables. The System Catalog is
called the data dictionary in other relational database management systems. It basically
contains system information that should not be modified or deleted; otherwise the database
will not work correctly. Table space TEMPSPACE1 is used by DB2 when it needs additional
space to perform some operations such as sorts. Table space USERSPACE1 is normally
used to store user database tables if there is no table space specified when creating a
table.
You can also create your own table spaces using the CREATE TABLESPACE statement.
Figure 4.9 shows the table space MYTBLS1 created inside database MYDB1 on instance
DB2. When you create a table space, you specify the disks to use and the memory (buffer
pool) to use. Therefore, if you have a “hot” table, that is, a table that is used very often, you
can allocate the fastest disks and the most memory by assigning a table space with these
characteristics.
In Figure 4.9, we show two other objects created by default: A buffer pool called
IBMDEFAULTBP, and the log files.
A buffer pool is basically a memory cache used by the database. You can create one or
more buffer pools, but there should always be one buffer pool with a page size that
matches the page size of existing table spaces. Pages and page size will be discussed in
more detail in Chapter 6, DB2 Architecture.
The log files are used for recovery. When you work on a database, not only is information
stored in the disks for the database, but while you are working on the database, log files
store all the operations executed on the data. Think of logs as temporary files where an
“autosave” operation is performed. Logs are discussed in more detail in Chapter 11,
Backup and Recovery.