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Oracle体系结构及备份(五)sga

时间:2021-07-01 10:21:17 帮助过:31人阅读

一 什么是SGA(参考 Oracle内存结构--SGA) SGA 是一组为系统分配的共享的内存结构,可以包含一个数据库实例的数据或控制信息。如果多个用户连接到同一个数据库实例,在实例的 SGA 中,数据可以被多个用户共享。当数据库实例启动时, SGA 的内存被自动分配;

一 什么是SGA(参考 Oracle内存结构--SGA)

SGA是一组为系统分配的共享的内存结构,可以包含一个数据库实例的数据或控制信息。如果多个用户连接到同一个数据库实例,在实例的SGA中,数据可以被多个用户共享。当数据库实例启动时,SGA的内存被自动分配;当数据库实例关闭时,SGA内存被回收。 SGA是占用内存最大的一个区域,同时也是影响数据库性能的重要因素。

SGA区是可读写的。所有登录到实例的用户都能读取SGA中的信息,而在oracle做执行操作时,服务进程会将修改的信息写入SGA区。

SGA主要包括了以下的数据结构:

数据缓冲(Buffer Cache

重做日志缓冲(Redo Log Buffer

共享池(Shared Pool

Java池(Java Pool

大池(Large Pool

流池(Streams Pool --- 10g以后才有)

数据字典缓存(Data Dictionary Cache

其他信息(如数据库和实例的状态信息)

The SGA is the Oracle structure that is located in shared memory. It contains static data structures, locks, and data buffers. Sufficient shared memory must be available to each Oracle process to address the entire SGA.

The maximum size of a single shared memory segment is specified by the shmmax kernel parameter.

The following table shows the recommended value for this parameter, depending on the platform:

If the size of the SGA exceeds the maximum size of a shared memory segment (shmmax or shm_max), then Oracle Database attempts to attach more contiguous segments to fulfill the requested SGA size. The shmseg kernel parameter specifies the maximum number of segments that can be attached by any process. Set the following initialization parameters to control the size of the SGA:

DB_CACHE_SIZE

DB_BLOCK_SIZE

JAVA_POOL_SIZE

LARGE_POOL_SIZE

LOG_BUFFERS

SHARED_POOL_SIZE

Alternatively, set the SGA_TARGET initialization parameter to enable automatic tuning of the SGA size.

Use caution when setting values for these parameters. When values are set too high, too much of the physical memory is devoted to shared memory. This results in poor performance.

An Oracle Database configured with Shared Server requires a higher setting for the SHARED_POOL_SIZE initialization parameter, or a custom configuration that uses the LARGE_POOL_SIZE initialization parameter. If you installed the database with Oracle Universal Installer, then the value of the SHARED_POOL_SIZE parameter is set automatically by Oracle Database Configuration Assistant. However, if you created a database manually, then increase the value of the SHARED_POOL_SIZE parameter in the parameter file by 1 KB for each concurrent user.

二 内存管理方式

Oracle 8i:手动 PGA管理

Oracle 9i:自动PGA内存管理、手工共享内存管理

Oracle 10g:自动共享内存管理

Oracle 11g:自动内存管理

三 SGA的分配

SGA的管理又三种方式

8iSGA的总大小由所有内存组件大小之和决定,不能直接定义SGA大小。对内部组件大小的修改必须在数据库重启后才能生效,所以叫SGA的静态管理。

9iSGA总大小由初始化参数SGA_MAX_SIZE确定,各个内存最贱大小之和不能超过这个参数,在这个大小之下,SGA各个内存组件可以在不重启数据库的情况下直接修改大小,所以叫做SGA的动态管理。

10gSGA大小既可以像9i一样动态管理,也可以实施SGA的自动管理,只需要设置初始化参数SGA_TARGETSGA各个内存组件就可以由数据库自动设置大小,设置的数据来源于系统自动收集的统计信息。在9i以后,SGA的内部组件大小可以动态调整,也可以由数据库自动管理,在设置内存大小的时候,分配的基本单位是粒度(granule.Granule是一段连续的虚拟内存,大小取决于SGA_MAX_SIZE的大小,如果SGA_MAX_SIZE小于128MGranule4M,否则Granule16M。各个内存组件分配大小必须是Granule的整倍数。整个SGA最小不小于3Granule大小。

9i中的规则如下

Linux/Unix

SGA <= 128M granule 4M

SGA > 128M granule 16M

Windows

SGA <= 128M granule 4M

SGA > 128M granule 8M

10G中的分配规则为

Linux/Unix/Windows

SGA <= 1G granule4M,否则为16M

SGA的各个组件大小是可以动态调整的,总大小不超过参数SGA_MAX_SIZE或者SGA_TARGET的大小。

四 操作示例


SQL> SELECT name, bytes/1024/1024 MB, resizeable from v$sgainfo;

NAME					 MB RES
-------------------------------- ---------- ---
Granule Size				  4 No
-------------------------------- ---------- ---

SQL> show parameter sga_max_size;

NAME				     TYPE	 VALUE
------------------------------------ ----------- ------------------------------
sga_max_size			     big integer 160M

SQL> ALTER SYSTEM SET sga_max_size=1025m scope=spfile;

System altered.

SQL> startup force;
ORACLE instance started.

Total System Global Area 1090519040 bytes
Fixed Size		    1218944 bytes
Variable Size		 1056966272 bytes
Database Buffers	   16777216 bytes
Redo Buffers		   15556608 bytes
Database mounted.
Database opened.
SQL> show parameter sga_max_size;

NAME				     TYPE	 VALUE
------------------------------------ ----------- ------------------------------
sga_max_size			     big integer 1040M
--因各个内存组件分配大小必须是Granule的整倍数,右1025隔1040最近,所以此处为1040M。

SQL> SELECT name, bytes/1024/1024 MB, resizeable from v$sgainfo;

NAME					 MB RES
-------------------------------- ---------- ---
Granule Size				 16 No
-------------------------------- ---------- ---

SQL> desc v$sgainfo;
 Name					   Null?    Type
 ----------------------------------------- -------- ----------------------------
 NAME						    VARCHAR2(32)
 BYTES						    NUMBER
 RESIZEABLE					    VARCHAR2(3)

SQL> select * from v$sgainfo;

NAME				      BYTES RES
-------------------------------- ---------- ---
Fixed SGA Size			    1218316 No
Redo Buffers			    2973696 No
Buffer Cache Size		   92274688 Yes
Shared Pool Size		   62914560 Yes
Large Pool Size 		    4194304 Yes
Java Pool Size			    4194304 Yes
Streams Pool Size			  0 Yes
Granule Size			    4194304 No
Maximum SGA Size		  167772160 No
Startup overhead in Shared Pool    37748736 No
Free SGA Memory Available		  0

11 rows selected.

SQL> SELECT name, bytes/1024/1024 MB, resizeable from v$sgainfo;

NAME					 MB RES
-------------------------------- ---------- ---
Fixed SGA Size			 1.16187668 No
Redo Buffers			  2.8359375 No
Buffer Cache Size			 88 Yes
Shared Pool Size			 60 Yes
Large Pool Size 			  4 Yes
Java Pool Size				  4 Yes
Streams Pool Size			  0 Yes
Granule Size				  4 No
Maximum SGA Size			160 No
Startup overhead in Shared Pool 	 36 No
Free SGA Memory Available		  0

11 rows selected.

SQL> show parameter sga_max_size;

NAME				     TYPE	 VALUE
------------------------------------ ----------- ------------------------------
sga_max_size			     big integer 160M
SQL> ALTER SYSTEM SET sga_max_size=1025m scope=spfile;

System altered.

SQL> startup force;
ORACLE instance started.

Total System Global Area 1090519040 bytes
Fixed Size		    1218944 bytes
Variable Size		 1056966272 bytes
Database Buffers	   16777216 bytes
Redo Buffers		   15556608 bytes
Database mounted.
Database opened.
SQL> show parameter sga_max_size;

NAME				     TYPE	 VALUE
------------------------------------ ----------- ------------------------------
sga_max_size			     big integer 1040M

SQL> SELECT name, bytes/1024/1024 MB, resizeable from v$sgainfo;

NAME					 MB RES
-------------------------------- ---------- ---
Fixed SGA Size			 1.16247559 No
Redo Buffers			 14.8359375 No
Buffer Cache Size			 16 Yes
Shared Pool Size			 96 Yes
Large Pool Size 			 16 Yes
Java Pool Size				 16 Yes
Streams Pool Size			  0 Yes
Granule Size				 16 No
Maximum SGA Size		       1040 No
Startup overhead in Shared Pool 	 32 No
Free SGA Memory Available		880

11 rows selected.

SQL> show parameter shared_p;

NAME				     TYPE	 VALUE
------------------------------------ ----------- ------------------------------
shared_pool_reserved_size	     big integer 4M
shared_pool_size		     big integer 0
SQL> ALTER SYSTEM SET shared_pool_size=10m;

System altered.

SQL> ALTER SYSTEM SET sga_max_size=800m scope=spfile;                   

System altered.

SQL> startup force;
ORACLE instance started.

Total System Global Area  838860800 bytes
Fixed Size		    1222192 bytes
Variable Size		  734005712 bytes
Database Buffers	  100663296 bytes
Redo Buffers		    2969600 bytes
Database mounted.
Database opened.
SQL> ALTER SYSTEM set shared_pool_size=10m;

System altered.

SQL> show parameter shared_p;

NAME				     TYPE	 VALUE
------------------------------------ ----------- ------------------------------
shared_pool_reserved_size	     big integer 838860
shared_pool_size		     big integer 12M
SQL> show parameter sga_max_size;

NAME				     TYPE	 VALUE
------------------------------------ ----------- ------------------------------
sga_max_size			     big integer 800M
SQL> show parameter sga_target;

NAME				     TYPE	 VALUE
------------------------------------ ----------- ------------------------------
sga_target			     big integer 160M
SQL> show parameter shared_pool_size;

NAME				     TYPE	 VALUE
------------------------------------ ----------- ------------------------------
shared_pool_size		     big integer 12M
SQL> show parameter large_pool_size;

NAME				     TYPE	 VALUE
------------------------------------ ----------- ------------------------------
large_pool_size 		     big integer 0
SQL> show parameter java_pool_size;

NAME				     TYPE	 VALUE
------------------------------------ ----------- ------------------------------
java_pool_size			     big integer 0
SQL> show parameter streams_pool_size; 

NAME				     TYPE	 VALUE
------------------------------------ ----------- ------------------------------
streams_pool_size		     big integer 0
SQL> show parameter db_cache_size;

NAME				     TYPE	 VALUE
------------------------------------ ----------- ------------------------------
db_cache_size			     big integer 0
SQL> show parameter sga_target;

NAME				     TYPE	 VALUE
------------------------------------ ----------- ------------------------------
sga_target			     big integer 160M
SQL>


五 总结

1. SGA是一组为系统分配的共享的内存结构,可以包含一个数据库实例的数据或控制信息。

2. SGA主要包括了以下的数据结构:数据缓冲(Buffer Cache)、重做日志缓冲(Redo Log Buffer)、共享池(Shared Pool)、Java池(Java Pool)、大池(Large Pool)、流池(Streams Pool --- 10g以后才有)、数据字典缓存(Data Dictionary Cache)、其他信息(如数据库和实例的状态信息);

3.可以通过v$sgainfo视图查看sga相关信息。



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