Database Reference
In-Depth Information
Offline Archive Log Files
The online redo log files are automatically moved to an offline device when the Oracle data-
base is configured to turn on the archive log mode. When needed, offline archive log files
can be used to perform a database point-in-time recovery.
Instance
When an Oracle Database is started it will start the background processes and open the
memory structures configured for the database, and this is referred to as an Oracle instance.
The Oracle instance is associated with the physical static database files to manage the data-
base operations.
Memory Structures
Memory structures such as the system global area (SGA) are opened at the Oracle instance
start and are based on the configuration parameter information. Refer to
Figure 19-1
while
studying the structures described in the following sections.
System Global Area
The system global area is a shared memory region allocated to an Oracle instance based on
the configuration parameters. The SGA has two primary subareas: the database buffer cache
and the redo log buffer. The Oracle SQL command “show sga” displays the current config-
uration of the SGA in the Oracle Database. This information can be obtained by using the
following commands after you log in to the system at the operating system level as ora<sid>
user:
Database Buffer Cache
The database buffer cache is a set of database buffers that keep the
modified and unmodified blocks of data in the memory area to facilitate faster access to it.
