ORACLE ASMM与AMM的总结

 

概念对比介绍

 

相信有些人会对ORACLE当中的AMM（Automatic Memory Management）与ASMM（Automatic Shared Memory Management）有些迷惑或混淆，不清楚两者的异同，本文会从几个方面来总结一下两者的异同。如有不足或疏漏之处，敬请指正！

 

从ORACLE发布的版本历史（时间轴）来看，ORACLE的内存管理的大致历程如下：

 

ORACLE  9i       PGA自动管理，SGA手动管理

ORACLE 10g      PGA自动管理，SGA自动管理（ASMM，自动共享内存管理）

ORACLE 11g      PGA,SGA统一自动管理（AMM，自动内存管理）

ORACLE 12c      跟11g一样，没有变化

 

 

官方文档的介绍资料如下：

 

·         Oracle 9i

 

·         Beginning with Oracle9i, the dynamic SGA infrastructure allowed for the sizing of the Buffer Cache, Shared Pool and the Large Pool without having to shut down the database. Key features being:

o   Dynamic Memory resizing

o   DB_CACHE_SIZE instead of DB_BLOCK_BUFFERS

o   DB_nK_CACHE_SIZE for multiple block sizes

o   PGA_AGGREGATE_TARGET Introduction of Automatic PGA Memory management

 

·         Oracle Database 10g

 

·         Automatic Shared Memory Management (ASMM) was introduced in 10g. You enable the automatic shared memory management feature by setting the SGA_TARGET parameter to a non-zero value.

 

·         Oracle Database 11g

 

·         Automatic Memory Management is being introduced in 11g. This enables automatic tuning of PGA and SGA with use of two new parameters named MEMORY_MAX_TARGET and MEMORY_TARGET

·         Oracle Database 12c

            Automatic Memory Management keeps the same behaviour as in 11g.

 

 

自动共享内存管理（Automatic Shared Memory Management ASMM）是ORACLE 10g开始引入的的新技术，ASMM用来实现SGA的自动管理。。当启用自动共享内存管理后，不再需要为每个内存组件设定值，当然如果你设置SGA_TARGET的同时，设置了db_cache_size、shared_pool_size这些参数，那么db_cache_size、shared_pool_size这些参数值会作为最小值要求。官方关于Automatic Shared Memory Management的介绍如下：

 

Automatic Shared Memory Management

 

In previous database releases, a database administrator (DBA) was required to manually specify different SGA component sizes by setting a number of initialization parameters, including the SHARED_POOL_SIZE, DB_CACHE_SIZE, JAVA_POOL_SIZE, and LARGE_POOL_SIZE parameters. Oracle Database 10g includes the Automatic Shared Memory Management feature which simplifies the SGA memory management significantly. In Oracle Database 10g, a DBA can simply specify the total amount of SGA memory available to an instance using the SGA_TARGET initialization parameter and the Oracle Database will automatically distribute this memory among various subcomponents to ensure most effective memory utilization.

 

When automatic SGA memory management is enabled, the sizes of the different SGA components are flexible and can adapt to the needs of a workload without requiring any additional configuration. The database automatically distributes the available memory among the various components as required, allowing the system to maximize the use of all available SGA memory.

 

ORACLE 10G版本开始推出了ASMM，自动SGA管理，它的出现一定程度上帮助DBA解决了管理SGA的问题，通过设置参数SGA_TARGET来控制ASMM，其中SGA_TARGET为零表示禁用ASMM，非零值表示启用ASMM。但是在10G R1等早期版本，ASMM还不够成熟，而且存在比较多的BUG，导致了比较多的问题。在ORACLE 10g R2后续版本中，ASMM才逐渐完善并成熟。

 

到了11g之后，ORACLE又实现了PGA和SGA的统一自动管理 ，这个称之为自动化内存管理(Automatic Memory Management,AMM)。从这些演化历程来看，ORACLE从最开始的手动配置各个组件参数，慢慢逐渐向智能化、傻瓜化、自动化的方向稳步前进。这个是一个必然的历史趋势。关于AMM的官方文档介绍如下：

 

About Automatic Memory Management

 

The simplest way to manage instance memory is to allow the Oracle Database instance to automatically manage and tune it for you. To do so (on most platforms), you set only a target memory size initialization parameter (MEMORY_TARGET) and optionally a maximum memory size initialization parameter (MEMORY_MAX_TARGET). The instance then tunes to the target memory size, redistributing memory as needed between the system global area (SGA) and the instance program global area (instance PGA). Because the target memory initialization parameter is dynamic, you can change the target memory size at any time without restarting the database. The maximum memory size serves as an upper limit so that you cannot accidentally set the target memory size too high, and so that enough memory is set aside for the Oracle Database instance in case you do want to increase total instance memory in the future. Because certain SGA components either cannot easily shrink or must remain at a minimum size, the instance also prevents you from setting the target memory size too low.

 

If you create your database with Database Configuration Assistant (DBCA) and choose the basic installation option, automatic memory management is enabled. If you choose advanced installation, Database Configuration Assistant (DBCA) enables you to select automatic memory management.

 

 

 

ORACLE 11g AMM 的引入， 组合出来有 5 种内存管理形式.

 

    自动内存管理（AMM）   : memory_target=非0，是自动内存管理，如果初始化参数 LOCK_SGA=TRUE，则 AMM 是不可用的。

    自动共享内存管理(ASMM): 在memory_target=0 and sga_target为非0的情形下是自动内存管理

    手工共享内存管理      : memory_target=0 and sga_target=0  指定 share_pool_size 、db_cache_size 等 sga 参数

    自动 PGA 管理         : memory_target=0 and workarea_size_policy=auto  and PGA_AGGREGATE_TARGET=值

    手动 PGA 管理         : memory_target=0 and workarea_size_policy=manal  然后指定 SORT_AREA_SIZE 等 PGA 参数，一般不使用手动管理PGA。

 

 

Oracle Database 11g supports various memory management methods, which are chosen by initialization parameter settings. Oracle recommends that you enable the automatic memory management method.

 

 

1. Automatic Memory Management – For Both the SGA and Instance PGA

2. Automatic Shared Memory Management – For the SGA

3. Manual Shared Memory Management – For the SGA

4. Automatic PGA Memory Management –For the Instance PGA

5. Manual PGA Memory Management – For the Instance PGA

 

 

 

 

 

ASMM切换到AMM

 

如下所示，当前实验环境下自动内存管理已被禁用（memory_target=0）

 

SQL> select * from v$version;

BANNER

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

Oracle Database 11g Enterprise Edition Release 11.2.0.1.0 - 64bit Production

PL/SQL Release 11.2.0.1.0 - Production

CORE    11.2.0.1.0      Production

TNS for Linux: Version 11.2.0.1.0 - Production

NLSRTL Version 11.2.0.1.0 - Production

SQL> show parameter memory_target ;

NAME                                 TYPE        VALUE

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

memory_target                        big integer 0

SQL> show parameter memory_max_target;

NAME                                 TYPE        VALUE

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

memory_max_target                    big integer 0

SQL> 

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

 

 

在11g中，如果使用ASMM，对应的内存共享段是真实的共享段。

 

SQL> !

[oracle@DB-Server ~]$ ipcs -m

------ Shared Memory Segments --------

key        shmid      owner      perms      bytes      nattch     status      

0x00000000 4128770    root      644        80         2                       

0x00000000 4161540    root      644        16384      2                       

0x00000000 4194309    root      644        280        2                       

0xfc5d1940 7012369    oracle    660        1075838976 49     

 

 

如下所示，首先检查参数文件类型，然后修改参数sga_target、memory_max_target、memory_target。因为当中有些参数为静态参数，所以在修改参数后，需要重启数据库。

 

 

 

SQL> show parameter spfile;

NAME                                 TYPE        VALUE

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

spfile                               string      /u01/app/oracle/product/11.1.0

                                                 /dbhome_1/dbs/spfilegsp.ora

SQL> alter system set sga_max_size=0 scope=spfile;

System altered.

SQL> alter system set sga_target=0 scope=spfile;

System altered.

SQL> alter system set pga_aggregate_target=0 scope=spfile;

SQL> alter system set memory_max_target=1G scope=spfile;

System altered.

SQL> alter system set memory_target=1G scope=spfile;

System altered.

SQL> 

 

重启数据库后，检查对应参数。

 

SQL> shutdown immediate;

Database closed.

Database dismounted.

ORACLE instance shut down.

SQL> startup

ORACLE instance started.

Total System Global Area  517816320 bytes

Fixed Size                  2214776 bytes

Variable Size             159384712 bytes

Database Buffers          348127232 bytes

Redo Buffers                8089600 bytes

Database mounted.

SQL> show parameter memory

NAME                                 TYPE        VALUE

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

hi_shared_memory_address             integer     0

memory_max_target                    big integer 1G

memory_target                        big integer 1G

shared_memory_address                integer     0

SQL> 

 

自动内存管理（AMM）启动之后，系统共享段变为“虚拟”共享段。

 

[oracle@DB-Server ~]$ ipcs -m

------ Shared Memory Segments --------

key        shmid      owner      perms      bytes      nattch     status      

0x00000000 4128770    root      644        80         2                       

0x00000000 4161540    root      644        16384      2                       

0x00000000 4194309    root      644        280        2                       

0xfc5d1940 7077905    oracle    660        4096       0            

 

 

 

 

 

11g MEMORY_TARGET Parameter Dependency

 

那么有个疑问，自动内存管理（AMM）模式下面，sga_max_size、sga_target、memory_max_target、memory_target、pga_aggregate_target这几个参数的关系是什么样的呢？其实官方文档已经有详细阐述

 

If MEMORY_TARGET is set to a non-zero value:

 

·         If SGA_TARGET and PGA_AGGREGATE_TARGET are set, they will be considered the minimum values for the sizes of SGA and the PGA respectively. MEMORY_TARGET values can range from SGA_TARGET + PGA_AGGREGATE_TARGET to MEMORY_MAX_TARGET.

·         If SGA_TARGET is set and PGA_AGGREGATE_TARGET is not set, we will still auto-tune both parameters. PGA_AGGREGATE_TARGET will be initialized to a value of MEMORY_TARGET - SGA_TARGET.

·         If PGA_AGGREGATE_TARGET is set and SGA_TARGET is not set, we will still auto-tune both parameters. SGA_TARGET will be initialized to the minimum non-zero value of MEMORY_TARGET - PGA_AGGREGATE_TARGET and SGA_MAX_SIZE and will auto tune its components.

·         If neither is set, they will be auto-tuned without any minimum or default values. We will have a policy of distributing the total memory set by MEMORY_TARGET parameter in a fixed ratio to the the SGA and PGA during initialization. The policy is to give 60% to the SGA and 40% to the PGA at startup.

 

If MEMORY_MAX_TARGET has not been explicitly set, but MEMORY_TARGET has, the instance automatically sets MEMORY_MAX_TARGET to the same value as MEMORY_TARGET. If MEMORY_TARGET has not been explicitly set, but MEMORY_MAX_TARGET has, then MEMORY_TARGET defaults to 0. After instance startup, it then is possible to dynamically change MEMORY_TARGET to a non-zero value, provided that it does not exceed the value of MEMORY_MAX_TARGET.

 

If MEMORY_TARGET is not set or set to set to 0 explicitly (default value is 0 for 11g):

·         If SGA_TARGET is set we will only auto-tune the sizes of the components of the SGA. PGA will be autotuned independent of whether it is explicitly set or not. However, the combination of SGA and PGA will not be auto-tuned, i.e. the SGA and PGA will not share memory and resize as with the case of MEMORY_TARGET being set to a non-zero value.

·         If neither SGA_TARGET nor PGA_AGGREGATE_TARGET is set, we will follow the same policy as we have today; PGA will be auto-tuned and the SGA will not be auto-tuned and parameters for some of the SGA components will have to be set explicitly (for SGA_TARGET).

·         If only MEMORY_MAX_TARGET is set, MEMORY_TARGET will default to 0 and we will not auto tune the SGA and PGA. It will default to 10gR2 behavior.

·         If SGA_MAX_SIZE is not user set, it is internally set to MEMORY_MAX_TARGET.

 

我们下面还是通过实验一一验证一下：

 

1:当MEMORY_TARGET大于0的情况下，可以设置SGA_TARGET、PGA_AGGREGATE_TARGET的值为非0，对应的意义分别如下：

 

    如果设置了SGA_TARGET和PGA_AGGREGATE_TARGET，它们分别表示SGA的的最小值和PGA的最小值。MEMORY_TARGET值的范围可以从SGA_TARGET + PGA_AGGREGATE_TARGET到MEMORY_MAX_TARGET。

    当然SGA_TARGET + PGA_AGGREGATE_TARGET的和必须小于等于memory_target，另外，如果同时设置了sga_target、pga_aggregate_target的值，memory_target的值必须大于等于sga_target与pga_aggregate_target之和。如下测试所示：

 

SQL> alter system set pga_aggregate_target=200m scope=both;

System altered.

SQL> show parameter pga_aggregate_target;

NAME                                 TYPE        VALUE

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

pga_aggregate_target                 big integer 200M

SQL> 

SQL> show parameter sga;

NAME                                 TYPE        VALUE

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

lock_sga                             boolean     FALSE

pre_page_sga                         boolean     FALSE

sga_max_size                         big integer 496M    #后面讲述为什么sga_max_size为什么没有被置为0

sga_target                           big integer 0

SQL> alter system set sga_target=400m scope=both;

System altered.

SQL> show parameter sga;

NAME                                 TYPE        VALUE

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

lock_sga                             boolean     FALSE

pre_page_sga                         boolean     FALSE

sga_max_size                         big integer 496M

sga_target                           big integer 400M

SQL> 

 

 

 

SQL> show parameter sga;

NAME                                 TYPE        VALUE

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

lock_sga                             boolean     FALSE

pre_page_sga                         boolean     FALSE

sga_max_size                         big integer 496M

sga_target                           big integer 400M

SQL> show parameter pga_aggregate_target;

NAME                                 TYPE        VALUE

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

pga_aggregate_target                 big integer 200M

SQL> alter system set memory_target=696m scope=both;

System altered.

SQL> alter system set memory_target=600m scope=both;

System altered.

SQL> alter system set memroy_target=500m scope=both;

alter system set memroy_target=500m scope=both

                 *

ERROR at line 1:

ORA-02065: illegal option for ALTER SYSTEM

SQL>  alter system set memory_target=500m scope=both;

 alter system set memory_target=500m scope=both

*

ERROR at line 1:

ORA-02097: parameter cannot be modified because specified value is invalid

ORA-00838: Specified value of MEMORY_TARGET is too small, needs to be at least 600M

 

 

 

SQL> show parameter memory_target;

NAME                                 TYPE        VALUE

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

memory_target                        big integer 600M

SQL> show parameter sga;

NAME                                 TYPE        VALUE

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

lock_sga                             boolean     FALSE

pre_page_sga                         boolean     FALSE

sga_max_size                         big integer 496M

sga_target                           big integer 400M

SQL> show parameter pga

NAME                                 TYPE        VALUE

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

pga_aggregate_target                 big integer 200M

SQL> 

SQL> 

SQL> alter system set pga_aggregate_target=201m scope=both;

alter system set pga_aggregate_target=300m scope=both

*

ERROR at line 1:

ORA-02097: parameter cannot be modified because specified value is invalid

ORA-00840: PGA_AGGREGATE_TARGET cannot be modified to the specified value

 

如上测试所示，如果设置了pga_aggregate_target和sga_target，那么pga_aggregate_target + sga_target 必须小于等于memory_target，另外，memory_target也必须大于等于（pga_aggregate_target + sga_target）

 

如果设置了SGA_TARGET并且PGA_AGGREGATE_TARGET没有设置，数据库仍然会自动调整这两个参数。 PGA_AGGREGATE_TARGET将被初始化为MEMORY_TARGET - SGA_TARGET的值。

如果设置了PGA_AGGREGATE_TARGET并且没有设置SGA_TARGET，数据库仍然会自动调整这两个参数。 SGA_TARGET将被初始化为MEMORY_TARGET - PGA_AGGREGATE_TARGET，并在这个值和SGA_MAX_SIZE这个区间范围内自动调整

如果SGA_TARGET和PGA_AGGREGATE_TARGET两者都没有设置的话，则它们将被自动调谐而没有任何最小值或默认值。 我们将有一个策略，在初始化过程中，将由MEMORY_TARGET参数设置的总内存以固定的比例分配给SGA和PGA。 政策是在启动时给予SGA 60%和PGA 40%给PGA。

 

 

2: 没有设置SGA_MAX_SIZE，但是为什么SGA_MAX_SIZE一直有值，即使将其设置为0或使用reset alter system set sga_max_size=0 scope=spfile; SGA_MAX_SIZE一直有值。官方关于SGA_MAX_SIZE的介绍如下：

 

 

SGA_MAX_SIZE specifies the maximum size of the SGA for the lifetime of the instance.

On 64-bit platforms and non-Windows 32-bit platforms, when either MEMORY_TARGET or MEMORY_MAX_TARGET is specified, the default value of SGA_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

也就是说在64位平台和非Windows 32位平台上，当指定MEMORY_TARGET或MEMORY_MAX_TARGET时，SGA_MAX_SIZE的默认值将设置为两个参数中较大的一个。 这导致更多的地址空间被保留用于SGA的扩展。 这也是之前一直让我迷惑的地方。另外，官方文档建议：当切换到AMM，即使用MEMORY_TARGET时，不应该设置参数SGA_MAX_SIZE（用于ASMM），因为这样做会修复SGA的大 小，因此与MEMORY_TARGET的预期用法相冲突。（原文：Check also for SGA_MAX_SIZE being set. When switching to AMM, i.e. using MEMORY_TARGET, the parameter SGA_MAX_SIZE (used for ASMM) should not be set as doing so fixes the size of the SGA, and hence conflicts with the intended use of MEMORY_TARGET.）

 

   

SGA_MAX_SIZE的值最好不要去修改，如果其值大于MEMORY_MAX_TARGET的话，就报ORA-00844 & ORA-00851 错误。如下所示

 

SQL> alter system set sga_max_size=1025M scope=spfile;

System altered.

SQL> shutdown immediate;

Database closed.

Database dismounted.

ORACLE instance shut down.

SQL> startup

ORA-00844: Parameter not taking MEMORY_TARGET into account

ORA-00851: SGA_MAX_SIZE 1090519040 cannot be set to more than MEMORY_TARGET 637534208.

SQL> 

 

 

此时需要生成对应spfile的pfile文件，然后找到*.sga_max_size这个值，删除后重新生成对应的spfile，启动数据库实例即可，当然你也可以设置其值大于MEMORY_TARGET即可。

 

 

AMM切换到ASMM

 

SQL> show parameter target;

NAME                                 TYPE        VALUE

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

archive_lag_target                   integer     0

db_flashback_retention_target        integer     1440

fast_start_io_target                 integer     0

fast_start_mttr_target               integer     0

memory_max_target                    big integer 1552M

memory_target                        big integer 1552M

parallel_servers_target              integer     16

pga_aggregate_target                 big integer 0

sga_target                           big integer 0

SQL> alter system set memory_max_target=0 scope=spfile;

System altered.

SQL> alter system set memory_target=0 scope=spfile;

System altered.

SQL> alter system set pga_aggregate_target=200m scope=spfile;

System altered.

SQL> alter system set sga_max_size=1g scope=spfile;

System altered.

SQL> alter system set sga_target=1g scope=spfile;

System altered.

SQL> 

SQL> startup

ORA-00843: Parameter not taking MEMORY_MAX_TARGET into account

ORA-00849: SGA_TARGET 1073741824 cannot be set to more than MEMORY_MAX_TARGET 0.

SQL> 

 

生成对应的spfile的pfile文件，然后删除memory_max_target和memory_target两个参数后，然后生成对应的spfile，最后重启数据库实例即可。

 

SQL> create pfile from spfile;

 

File created.

 

 

*.memory_max_target=0

*.memory_target=0

 

SQL> create spfile from pfile;

 

File created.

 

SQL>

 

 

选择AMM还是HugePages

 

ORACLE 11g开始推出AMM，它是ORACLE在ASMM的基础上的进一步内存管理自动化的演进。ASMM是自动管理SGA，而AMM则是将SGA与PGA联合起来自动管理、调整。只需要设置memory_target一个参数就可以完成整个数据库实例内存的配置。但是这个功能没被广泛使用，因为AMM最大的问题在于不能使用标准大页。有时候为了使用标准大页功能，可能有些系统会禁用AMM.那么到底是用AMM还是使用大页呢？很多人(大师)倾向使用大页功能而非AMM，关于这个可以参考下面博文

 

MEMORY_TARGET (SGA_TARGET) or HugePages – which to choose?

AMM and Linux Huge Pages

Oracle Memory Management and HugePage

 

 

如何从AMM切换到HugePage，可以参考官方文档（ID 2128928.1)

 

How To Convert A Database Using AMM (Automatic Memory Management) To A Database That Has Been Configured With Hugepage

 

 

参考资料：

 

http://blog.ronnyegner-consulting.de/2010/03/31/memory_target-sga_target-or-hugepages-which-to-choose/

https://docs.oracle.com/cd/E18283_01/server.112/e17110/initparams230.htm

http://www.dba-oracle.com/t_amm_automatic_memory.htm

https://docs.oracle.com/cd/B28359_01/server.111/b28310/memory003.htm#ADMIN11011

https://www.jianshu.com/p/9715280a4ced

http://oracle-help.com/oracle-database/relationship-memory_target-sga_target-pga_aggregate_target/

https://support.oracle.com/epmos/faces/DocumentDisplay?_afrLoop=275836827924018&id=443746.1&_afrWindowMode=0&_adf.ctrl-state=149nyur949_198