Oracle 11g自动内存管理(AMM)相关的初始化参数
Oracle对内存的管理越来越趋向智能化、自动化，从9i通过PGA_AGGREGATE_TARGET参数实现PGA的自动管理，10g通过Automatic Shared M
Oracle对内存的管理越来越趋向智能化、自动化，从9i通过PGA_AGGREGATE_TARGET参数实现PGA的自动管理，10g通过Automatic Shared Memory Management（ASMM）实现SGA的自动管理，到11g通过Automatic Memory Management（AMM）实现内存（SGA+PGA）的自动管理。由于11G(AMM)的引入，DBA只需要设置一个或两个参数就可以实现ORACLE对整个内存块的自动管理。memory_max_target参数指定了ORACLE可以分配的最大内存大小，如果不指定memory_target参数，，默认为0，即和memory_max_target同样大小。
默认情况下，Oracle 11g是使用AMM的。我们在安装过程中，指定Oracle使用内存的百分比，这个取值就作为MEMORY_TARGET和MEMORY_MAX_TARGET的初始取值使用。如果这两个参数设置为非零取值，那么Oracle就是采用AMM管理策略的。同时，如果我们设置这两个参数为0，则AMM自动关闭。对应的SGA_TARGET、PGA_AGGREGATE_TARGET参数取值非零之后，Oracle自动退化使用ASMM特性。
下面介绍一下4个相关的初始化参数。
SGA_MAX_SIZE
属性
描述
参数类型
Big integer
语法
SGA_MAX_SIZE = integer [K | M | G]
默认值
是否可修改
否
取值范围
0~视操作系统而定
SGA_TARGET
属性
描述
参数类型
Big integer
语法
SGA_TARGET = integer [K | M | G]
默认值
是否可修改
ALTER SYSTEM
取值范围
64M~视操作系统而定
是否基本参数
是
l Buffer cache (DB_CACHE_SIZE)
l Shared pool (SHARED_POOL_SIZE)
l Large pool (LARGE_POOL_SIZE)
l Java pool (JAVA_POOL_SIZE)
l Streams pool (STREAMS_POOL_SIZE)
l Log buffer
l Fixed SGA and other internal allocations
MEMORY_MAX_TARGET
属性
描述
参数类型
Big integer
语法
MEMORY _MAX_TARGET = integer [K | M | G]
默认值
0
是否可修改
NO
取值范围
0~Oracle数据库可用物理内存大小
是否基本参数
否
MEMORY_TARGET
属性
描述
参数类型
Big integer
语法
MEMORY _TARGET = integer [K | M | G]
默认值
是否可修改
ALTER SYSTEM
取值范围
152M~视操作系统而定
是否基本参数
否
MEMORY_TARGET指定Oracle系统范围的可用内存。数据库将内存大小调整为MEMORY_TARGET的值，根据需要减少或扩大SGA和PGA。
MEMORY_TARGET应该设置大于或等于当前SGA和PGA大小的总和。
在文本初始化参数文件中，如果省略MEMORY_MAX_TARGET并包括一个MEMORY_TARGET值，则数据库会自动将MEMORY_MAX_TARGET的值设置为MEMORY_TARGET的值。如果省略MEMORY_TARGET并包括MEMORY_MAX_TARGET，则MEMORY_TARGET参数默认值为零。在数据库启动后，你可以动态地修改MEMORY_TARGET为非零值，只要它不超过MEMORY_MAX_TARGET的值。
总的内存使用量可以超过MEMORY_TARGET的值。例如，只要在操作系统级别上有可用内存，无论MEMORY_TARGET的值是多少，都可以将内存分配给PL / SQL表和可变数组。
在“默认值”字段中，IMMEDIATE模式自动调整要求是必要的，这样可以避免ORA-04031错误。DEFERRED和IMMEDIATE模式的具体值，可以通过V$MEMORY_RESIZE_OPS视图的OPER_MODE列来查看。
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