Each of these physical storage structures is discussed later, in the section titled ???Oracle
Physical Storage Structures.???
Instances
The main components of a typical enterprise server are one or more CPUs, disk space, and
memory. Whereas the Oracle database is stored on a server??™s disk, an Oracle instance exists
in the server??™s memory. An Oracle instance is composed of a large block of memory allocated
in an area called the System Global Area (SGA), along with a number of background processes
that interact between the SGA and the database files on disk.
In an Oracle Real Application Cluster (RAC), more than one instance will use the same
database. Although the instances that share the database can be on the same server, most likely
the instances will be on separate servers that are connected by a high-speed interconnect and
access a database that resides on a specialized RAID-enabled disk subsystem. More details on
how a RAC installation is configured are provided in Chapter 10.
FIGURE 1-1 Sample database table
6 Oracle Database 11g DBA Handbook
Oracle Logical Storage Structures
The datafiles in an Oracle database are grouped together into one or more tablespaces. Within
each tablespace, the logical database structures, such as tables and indexes, are segments that are
further subdivided into extents and blocks. This logical subdivision of storage allows Oracle to
have more efficient control over disk space usage.
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