Oracle初始化参数之memory_target

一、引言：

    Oracle 9i引入pga_aggregate_target，可以自动对PGA进行调整；

    Oracle 10g引入sga_target，可以自动对SGA进行调整；

    Oracle 11g则对这两部分进行综合，引入memory_target，可以自动调整所有的内存，这就是新引入的自动内存管理特性。

二、本文说明：

      操作系统：rhel 5.4 x32

      数据库：oracle 11g r2

三、memory_target的介绍：

    3.1、下面通过示例了解一下memory_target的设置与PGA和SGA关系：

SQL> show parameter memory_target;NAME                     TYPE     VALUE
------------------------------------ ----------- ------------------------------
memory_target                 big integer 316M
SQL> show sga;Total System Global Area  330600448 bytes
Fixed Size            1336344 bytes
Variable Size          247466984 bytes
Database Buffers       75497472 bytes
Redo Buffers            6299648 bytes
SQL> alter system set memory_target=200m 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;System altered.SQL> shutdown immediate;
Database closed.
Database dismounted.
ORACLE instance shut down.
SQL> startup
ORACLE instance started.Total System Global Area  209235968 bytes
Fixed Size            1335528 bytes
Variable Size          201330456 bytes
Database Buffers        4194304 bytes
Redo Buffers            2375680 bytes
Database mounted.
Database opened.

    设置memory_target参数后，实际上Oracle会自动设置并调整一下两个参数来分配SGA和PGA的内存，这和Oracle 10g自动设置sga_target后分配db_cache_size和shared_pool_size的机制是一样的。

SQL> col ksppinm for a20;
SQL> col ksppstvl for a20;
SQL> select a.ksppinm name,b.ksppstvl value2     from x$ksppi a,x$ksppcv b3   where a.indx = b.indx4      and (a.ksppinm like '%sga_target%'5  or a.ksppinm like '%pga_aggregate_target%');NAME                                VALUE
-----------------------     -------------------
sga_target                           0
__sga_target                     142606336
pga_aggregate_target                 0
__pga_aggregate_target            67108864

    3.2、讨论一下11g中memory_target设置和不设置对SGA/PGA的影响：

    3.2.1、如果memory_target设置为非0值

      (下面有四种情况来对SGA和PGA的大小进行分配)

      3.2.1.1、sga_target和pga_aggregate_target已经设置大小

        如果Oracle中已经设置了参数sga_target和pga_aggregate_target，则这两个参数将各自被分配为最小值为他们的目标值。

        memory_target = sga_target + pga_aggregate_target，大小和memory_max_size一致。

      3.2.1.2、sga_target设置大小，pga_aggregate_target没有设置大小

        那么pga_aggregate_target初始化值=memory_target-sga_target

      3.2.1.3、sga_target没有设置大小，pga_aggregate_target设置大小

        那么sga_target初始化值=memory_target-pga_aggregate_target

      3.2.1.4、sga_target和pga_aggregate_target都没有设置大小

        Oracle 11g中对这种sga_target和pag_aggregate_target都没有设定大小的情况下，Oracle将对这两个值没有最小值和默认值。Oracle将根据数据库运行状况进行分配大小。但在数据库启动是会有一个固定比例来分配：

        sga_target = memory_target*60%

        pga_aggregate_target = memory_target*40%

    3.2.2、如果memory_target没有设置或 = 0(在11g中默认为0)

      11g中默认为0则初始状态下取消了memory_target的作用，完全和10g在内存管理上一致，完全向下兼容。(也有三种情况来对SGA和PGA的大小进行分配)

      3.2.2.1、sga_target设置值，则自动调节SGA中的shared pool,buffer cache,redo log buffer,java pool,larger pool等内存空间的大小。PGA则依赖pga_aggregate_target的大小。sga和pga不能自动增长和自动缩小。

      3.2.2.2、sga_target和pga_aggregate_target都没有设置

        SGA中的各组件大小都要明确设定，不能自动调整各组件大小。PGA不能自动增长和收缩。

      3.2.2.3、memory_max_target设置而memory_target = 0这种情况先和10g一样，不做说明。 

三、ORA-00845

    最后谈一下ORA-00845的由来和解决方案：

    如果memory_max_target/memory_target设置过大，可能导致instance无法启动，报ORA-00845错误。

[oracle@yft bin]$ oerr ora 00845
00845, 00000, "MEMORY_TARGET not supported on this system"
// *Cause: The MEMORY_TARGET parameter was not supported on this operating system or /dev/shm was not sized 
// *Action: Refer to documentation for a list of supported operating systems. Or, size /dev/shm to be at leacle instance running on the system.SQL> alter system set memory_max_target=400m scope=spfile;System altered.SQL> shutdown immediate;
Database closed.
Database dismounted.
ORACLE instance shut down.
SQL> startup
ORA-00845: MEMORY_TARGET not supported on this system

 这个错误有点误导，实际上这并不是说该平台版本上不支持AMM特性，只是设置的memory_max_target超过了系统中设置的share memory(/dev/shm)而已。

[oracle@yft bin]$ df -h
Filesystem            Size  Used Avail Use% Mounted on
tmpfs                 395M     0  395M   0% /dev/shm

在Oracle 11g for linux中似乎是用了一种新的机制来管理共享内存段，而不是传统的sys /dev/shm了。在alert.ora中可以找到更准确的错误描述：

Mon Feb 25 12:13:21 2013
Starting ORACLE instance (normal)
WARNING: You are trying to use the MEMORY_TARGET feature. This feature requires the /dev/shm file system to be mounted for at least 419430400 bytes. /dev/shm is either not mounted or is mounted with available space less than this size. Please fix this so that MEMORY_TARGET can work as expected. Current available is 413466624 and used is 0 bytes. Ensure that the mount point is /dev/shm for this directory.
memory_target needs larger /dev/shm

 

 解决的办法之一是增加tmpfs文件系统的容量:

      修改/etc/fstab中tmpfs对应的行；将原来的tmpfs   /dev/shm  tmpfs defaults 0 0 改成tmpfs /dev/shm tmpfs default,size=1024M 0 0,这样tmpfs增大为1G，重新mount /dev/shm使之生效。

[root@yft ~]# vi /etc/fstab
tmpfs                   /dev/shm                tmpfs   defaults,size=420m        0 0[root@yft ~]# mount -o remount /dev/shm
[root@yft ~]# df -h
Filesystem            Size  Used Avail Use% Mounted on
tmpfs                 420M     0  420M   0% /dev/shmSQL> startup
ORACLE instance started.Total System Global Area  418484224 bytes
Fixed Size            1336932 bytes
Variable Size          406849948 bytes
Database Buffers        4194304 bytes
Redo Buffers            6103040 bytes
Database mounted.