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Oracle 10g 库启动的SGA大小由sga_target与sga_max_size决定，分下面三种情况讨论
sga_target=sga_max_size
参数文件指定值
*.sga_target=599785472
启动 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
参数文件指定值 *.sga_max_size=800m
*.sga_target=700m
启动数据库 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 700M
sga_target>sga_max_size 参数文件指定值 *.sga_max_size=600m
*.sga_target=700m
而数据库实际启动的大小 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
实验结论
由以上三个实验可以看到，数据库启动时SGA的大小由SGA_TARGET和SGA_MAX_SIZE中的较大值决定
当SGA_TARGET <= SGA_MAX_SIZE时，以SGA_MAX_SIZE为准
当SGA_TARGET > SGA_MAX_SIZE时，将SGA_TARGET的值赋予SGA_MAX_SIZE，然后以SGA_MAX_SIZE为准
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