今天晚上新装一台Oracle 11g的数据库，打算将SGA设大一点，知道 11g 中有一个新特新 MEMORY_TARGET，于是尝一下鲜，谁知报了个 ORA-00845，报错比较容易迷惑人，不借助Google真得想半天：[size=1em][table=98%,inherit]
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[backcolor=inherit][backcolor=inherit]SQL> alter system set memory_max_target=3G scope=spfile ;

[backcolor=inherit]System altered.

[backcolor=inherit]SQL> alter system set memory_target=2G scope=spfile ;

[backcolor=inherit]System altered.

[backcolor=inherit]SQL>
[backcolor=inherit]SQL> shutdown immediate
[backcolor=inherit]Database closed.
[backcolor=inherit]Database dismounted.
[backcolor=inherit]ORACLE instance shut down.
[backcolor=inherit]SQL> startup ;
[backcolor=inherit]ORA-00845: MEMORY_TARGET not supported on this system

来自Oracle的官方解析是：Starting with Oracle Database 11g, the Automatic Memory Management feature requires more shared memory (/dev/shm)and file descriptors. The size of the shared memory should be at least the greater of MEMORY_MAX_TARGET and MEMORY_TARGET for each Oracle instance on the computer. If MEMORY_MAX_TARGET or MEMORY_TARGET is set to a non zero value, and an incorrect size is assigned to the shared memory, it will result in an ORA-00845 error at startup. 简单来说就是 MEMORY_MAX_TARGET 的设置不能超过 /dev/shm 的大小：[size=1em][table=98%,inherit]
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[backcolor=inherit][backcolor=inherit][oracle@FWDB FWDB]$ df -h | grep shm
[backcolor=inherit]tmpfs 2.0G 0 2.0G 0% /dev/shm

还真是撞到这个枪口上了： 马上把它加大：[size=1em][table=98%,inherit]
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[backcolor=inherit][backcolor=inherit][root@FWDB ~]# cat /etc/fstab | grep tmpfs
[backcolor=inherit]tmpfs /dev/shm tmpfs defaults,size=4G 0 0

现在可以通过重启使这个配置生效，也可以通过重新挂载来修改其大小：[size=1em][table=98%,inherit]
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[backcolor=inherit][backcolor=inherit][root@FWDB ~]# mount -o remount,size=4G /dev/shm
[backcolor=inherit][root@FWDB ~]# df -h | grep shm
[backcolor=inherit]tmpfs 4.0G 0 4.0G 0% /dev/shm

再次启动数据库，没有报错了。