See Chapter 8 for
more information on how to use the Redo Logfile Sizing Advisor.
FIGURE 1-4 Multiplexing redo log files
32 Oracle Database 11g DBA Handbook
Archived Redo Log Files
If the database is in archivelog mode, Oracle copies redo log files to a specified location before
they can be reused in the redo log switch cycle.
Oracle Memory Structures
Oracle uses the server??™s physical memory to hold many things for an Oracle instance: the Oracle
executable code itself, session information, individual processes associated with the database,
and information shared between processes (such as locks on database objects). In addition,
the memory structures contain user and data dictionary SQL statements, along with cached
information that is eventually permanently stored on disk, such as data blocks from database
segments and information about completed transactions in the database. The data area allocated
for an Oracle instance is called the System Global Area (SGA). The Oracle executables reside in
the software code area. In addition, an area called the Program Global Area (PGA) is private to
each server and background process; one PGA is allocated for each process. Figure 1-5 shows the
relationships between these Oracle memory structures.
System Global Area
The System Global Area is a group of shared memory structures for an Oracle instance, shared by
the users of the database instance. When an Oracle instance is started, memory is allocated for the
FIGURE 1-5 Oracle logical memory structures
Chapter 1: Getting Started with the Oracle Architecture 33
SGA based on the values specified in the initialization parameter file or hard-coded in the Oracle
software.
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