oracle三对内存参数间关系之3AMM启动和关闭两种情况下ASMM机制涉

ASMM机制涉及的一对参数为： SGA_TARGET和SGA_MAX_SIZE 。 SGA_TARGET Default value 0 (SGA autotuning is disabled for DEFERRED mode autotuning requests, but allowed for IMMEDIATE mode autotuning requests) Modifiable ALTER SYSTEM （即表示该参数

ASMM机制涉及的一对参数为：SGA_TARGET和SGA_MAX_SIZE。

SGA_TARGET

Default value	0 (SGA autotuning is disabled forDEFERRED mode autotuning requests, but allowed forIMMEDIATE mode autotuning requests)
Modifiable	ALTER SYSTEM（即表示该参数为动态参数）
Range of values	64 MB to operating system-dependent

参见：http://docs.oracle.com/cd/E11882_01/server.112/e40402/initparams236.htm#I1010256

SGA_MAX_SIZE

Default value	Initial size of SGA at startup, dependent on the sizes of different pools in the SGA, such as buffer cache, shared pool, large pool, and so on.
Modifiable	No（即表示该参数为静态参数）
Range of values	0 to operating system-dependent

On 64-bit platforms and non-Windows 32-bit platforms, when either MEMORY_TARGET orMEMORY_MAX_TARGET is specified, the default value ofSGA_MAX_SIZE is set to the larger of the two parameters. This causes more address space to be reserved for expansion of the SGA.

On Windows 32-bit platforms, the default value of SGA_MAX_SIZE is the largest of the following values:

• 60% of MEMORY_TARGET, if specified

• 60% of MEMORY_MAX_TARGET, if specified

• 25% of the total available virtual address space

参见：http://docs.oracle.com/cd/E11882_01/server.112/e40402/initparams235.htm

上述描述可知，

1、当SGA_TARGET=非0时，SGA_TARGET要在64 MB to operating system-dependent这个范围内，确切说是，SGA_TARGET要在64 MB toSGA_MAX_SIZE。

2、

由于SGA_TARGET始终SGA_MAX_SIZE