Oracle 10g 庫啟動的SGA大小由sga_target與sga_max_size決定，分下面三種情況討論
sga_target=sga_max_size
參數文件指定值
*.sga_target=599785472啟動
SQL> startup
ORACLE instance started.
Total System Global Area 599785472 bytes
Fixed Size 2022632 bytes
Variable Size 171967256 bytes
Database Buffers 423624704 bytes
Redo Buffers 2170880 bytes
Database mounted.
Database opened.sga_target < sga_max_size
參數文件指定值
*.sga_max_size=800m
*.sga_target=700m啟動數據庫
SQL> startup
ORACLE instance started.
Total System Global Area 838860800 bytes
Fixed Size 2024496 bytes
Variable Size 297798608 bytes
Database Buffers 536870912 bytes
Redo Buffers 2166784 bytes
Database mounted.
Database opened.SQL> select 838860800/1024/1024 from dual;
838860800/1024/1024
-------------------
800
SQL> show parameter sga_
NAME TYPE VALUE
------------------- ----------- -----------------------
sga_max_size big integer 800M
sga_target big integer 700Msga_target>sga_max_size
參數文件指定值
*.sga_max_size=600m
*.sga_target=700m而數據庫實際啟動的大小
SQL> startup
ORACLE instance started.
Total System Global Area 734003200 bytes
Fixed Size 2023656 bytes
Variable Size 192941848 bytes
Database Buffers 536870912 bytes
Redo Buffers 2166784 bytes
Database mounted.
Database opened.SQL> select 734003200/1024/1024 from dual;
734003200/1024/1024
-------------------
700
SQL> show parameter sga_
NAME TYPE VALUE
------------------------------------ ----------- ------------------------------
sga_max_size big integer 700M
sga_target big integer 700M實驗結論
由以上三個實驗可以看到，數據庫啟動時SGA的大小由SGA_TARGET和SGA_MAX_SIZE中的較大值決定
當SGA_TARGET <= SGA_MAX_SIZE時，以SGA_MAX_SIZE為准
當SGA_TARGET > SGA_MAX_SIZE時，將SGA_TARGET的值賦予SGA_MAX_SIZE，然後以SGA_MAX_SIZE為准
