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：

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SQL> alter system set memory_max_target=3G scope=spfile ;
 
System altered.
 
SQL> alter system set memory_target=2G scope=spfile ;     
 
System altered.
 
SQL>
SQL> shutdown immediate
Database closed.
Database dismounted.
ORACLE instance shut down.
SQL> startup ;
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：

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[oracle@FWDB FWDB]$ df  -h | grep  shm
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：

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[root@FWDB ~] # cat /etc/fstab | grep tmpfs
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：

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[root@FWDB ~] # mount -o remount,size=4G /dev/shm
[root@FWDB ~] # df -h | grep shm
tmpfs                 4.0G     0  4.0G   0% /dev/shm

      再次启动数据库，没有报错了。