Oracle 10g 如何调整 sga_max_size 与 sga_target

sga_max_size是相对于操作系统来讲的，当启动oracle时，一次性分配给oracle实例的sga不会超过sga_max_size值；而sga_target是相对于oracle这个正在运行的应用软件来说的，在系统已经分配了sga_max_size值的内存给oracle的前提下，对sga_target值进修改。对sga_max_size修改时，scope只对spfile生效。

　　第一次启动oracle时，sga_target与sga_max_size数值一样。

SQL> show parameter sga;

NAME
TYPE VALUE

------------------------------------ ---------------------------------------------

lock_sga boolean FALSE

pre_page_sga boolean FALSE

sga_max_size big integer 892M

sga_target big integer 892M

一、对sga_max_size提高、降低的限制

1、用户调高sga_max_size，不对sga_target进行修改，重启oracle时，sga_max_size会相应变化

SQL> alter system set sga_max_size=1024mscope=spfile;

System altered.

SQL> show parameter sga;

NAME TYPE VALUE

------------------------------------ ---------------------------------------------

lock_sga boolean FALSE

pre_page_sga boolean FALSE

sga_max_size big integer 892M

sga_target big integer 892M

SQL> shutdown immediate

Database closed.

Database dismounted.

ORACLE instance shut down.

SQL> startup

ORACLE instance started.

Total System Global Area 1073741824 bytes

Fixed Size 2026296 bytes

Variable Size 377488584 bytes

Database Buffers 687865856 bytes

Redo Buffers 6361088 bytes

Database mounted.

Database opened.

SQL> show parameter sga

NAME TYPE VALUE

------------------------------------ ---------------------------------------------

lock_sga boolean FALSE

pre_page_sga boolean FALSE

sga_max_size big integer 1G

sga_target big integer 892M

2、当用户调整sga_max_size的大小，使其小于sga_target，重新启动oracle的时候，系统分配给sga_max_size将参照sga_target值

SQL> alter system set sga_max_size=520mscope=spfile;

System altered.

SQL> show parameter sga

NAME TYPE VALUE

------------------------------------ ---------------------------------------------

lock_sga boolean FALSE

pre_page_sga boolean FALSE

sga_max_size big integer 1G

sga_target big integer 892M

SQL> shutdown immediate

startup

Database closed.

Database dismounted.

ORACLE instance shut down.

SQL> ORACLE instance started.

Total System Global Area 935329792 bytes

Fixed Size 2025168 bytes

Variable Size 243271984 bytes

Database Buffers 683671552 bytes

Redo Buffers 6361088 bytes

Database mounted.

Database opened.

SQL> show parameter sga

NAME TYPE VALUE

------------------------------------ ---------------------------------------------

lock_sga boolean FALSE

pre_page_sga boolean FALSE

sga_max_size big integer 892M

sga_target big integer 892M

二、对sga_target提高、降低的限制

3、想提高sga_target值使其大于sga_max_size，需先提高sga_max_size值，重启oracle后才能修改并提高sga_target值

SQL> show parameter sga

NAME TYPE VALUE

------------------------------------ ---------------------------------------------

lock_sga boolean FALSE

pre_page_sga boolean FALSE

sga_max_size big integer 892M

sga_target big integer 892M

SQL> alter system set sga_target=1024mscope=both;

alter system set sga_target=1024m scope=both

*

ERROR at line 1:

ORA-02097: parameter cannot be modified because specified value isinvalid

ORA-00823: Specified value of sga_target greater thansga_max_size

SQL> alter system set sga_max_size=1024mscope=spfile;

System altered.

SQL> alter system set sga_target=1024mscope=both;

alter system set sga_target=1024m scope=both

*

ERROR at line 1:

ORA-02097: parameter cannot be modified because specified value isinvalid

ORA-00823: Specified value of sga_target greater thansga_max_size

SQL> shutdown immediate

startup

Database closed.

Database dismounted.

ORACLE instance shut down.

SQL> ORACLE instance started.

Total System Global Area 1073741824 bytes

Fixed Size 2026296 bytes

Variable Size 385877192 bytes

Database Buffers 679477248 bytes

Redo Buffers 6361088 bytes

Database mounted.

Database opened.

SQL> show parameter sga

NAME TYPE VALUE

------------------------------------ ---------------------------------------------

lock_sga boolean FALSE

pre_page_sga boolean FALSE

sga_max_size big integer 1G

sga_target big integer 892M

SQL> alter system set sga_target=1gscope=both;

System altered.

SQL> show parameter sga

NAME TYPE VALUE

------------------------------------ ---------------------------------------------

lock_sga boolean FALSE

pre_page_sga boolean FALSE

sga_max_size big integer 1G

sga_target big integer 1G

总结：1、sga_max_size值调大、调小不受sga_target的限制，但若要让sga_max_size调小后的值生效，则必须先降低sga_target的大小，然后再修改sga_max_size，重启使其生效；

2、sga_target调高，必须在sga_max_size范围内，调小不受sga_max_size的限制。