SGA_TARGET vs SGA_MAX_SIZE
SGA_MAX_SIZE
sga_max_size sets the maximum value for sga_target
If sga_max_size is less than the sum of db_cache_size + log_buffer
+ shared_pool_size + large_pool_size at initialization time, then the value of
sga_max_size is ignored.
SGA_TARGET
This parameter is new with Oracle 10g. It specifies the total amaount
of SGA memory available to an instance. Setting this parameter makes Oracle
distribute the available memory among various components - such as shared pool
(for SQL and PL/SQL), Java pool, large_pool and buffer cache - as required.
This new feature is called Automatic Shared Memory Management. With
ASMM, the parameters java_pool_size, shared_pool_size, large_pool_size and
db_cache_size need not be specified explicitely anymore.
sga_target cannot be higher than sga_max_size.
SGA_TARGET is a database initialization parameter (introduced in
Oracle 10g) that can be used for automatic SGA memory sizing.
Parameter description:
SGA_TARGET
Property Description
Parameter type Big integer
Syntax SGA_TARGET = integer [K | M | G]
Default value 0 (SGA autotuning is disabled)
Modifiable ALTER SYSTEM
Range of values 64 to operating system-dependent
Basic Yes
SGA_TARGET provides the following:
• Single parameter for total SGA size
• Automatically sizes SGA components
• Memory is transferred to where most needed
• Uses workload information
• Uses internal advisory predictions
• STATISTICS_LEVEL must be set to TYPICAL
By using one parameter we don't need to use all other SGA
parameters like.
• DB_CACHE_SIZE (DEFAULT buffer pool)
• SHARED_POOL_SIZE (Shared Pool)
• LARGE_POOL_SIZE (Large Pool)
• JAVA_POOL_SIZE (Java Pool)
Enable SGA_TARGET
SQL> show parameter sga_target
NAME TYPE VALUE
------------------------------------ -----------
------------------------------
sga_target big integer 0
SQL> show parameter sga_max_size
NAME TYPE VALUE
------------------------------------ -----------
------------------------------
sga_max_size big integer 600M
As we can see out automatic SGA tuning is not enabled so we can
enable it by setting the SGA_TARGET parameter value.
SQL> alter system set sga_target=500m;
System altered.
SQL> show parameter sga_target
NAME TYPE VALUE
------------------------------------ ----------- -------
sga_target big integer 500M
Resize SGA_TARGET
• SGA_TARGET is dynamic
• Can be increased till SGA_MAX_SIZE
• Can be reduced till some component reaches minimum size
• Change in value of SGA_TARGET affects only automatically sized
components
SQL> show parameter sga_max_size
NAME TYPE VALUE
------------------------------------ ----------- -----------
sga_max_size big integer 600M
SQL> show parameter sga_target
NAME TYPE VALUE
------------------------------------ ----------- -----------
sga_target big integer 500M
WE can resize it to only 600m if we will try to increase it from
600m we will get error.
SQL> alter system set sga_target=605m;
alter system set sga_target=605m *
ERROR at line 1:
ORA-02097: parameter cannot be modified because specified value is
invalid
ORA-00823: Specified value of sga_target greater than sga_max_size
For that we must first increase our SGA_MAX_SIZE parameter
value.But we must restart out instance because its STATIC parameter.
SQL> alter system set sga_max_size=956 scope=spfile;
System altered.
SQL> startup force
ORACLE instance started.
Total System Global Area 1000189952 bytes
Fixed Size 1337492 bytes
Variable Size 624953196 bytes
Database Buffers 369098752 bytes
Redo Buffers 4800512 bytes
Database mounted.
Database opened.
SQL> show parameter sga_max_size
NAME TYPE VALUE
------------------------------------ ----------- --------
sga_max_size big integer 956M
SQL> alter system set sga_target=900m;
System altered.
Disable SGA_TARGET
We can Disable our automatic SGA tuning by setting the parameter
SGA_TARGET to value digit 0.
SQL> alter system set sga_target=0;
System altered.
