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：

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：

[oracle@FWDB FWDB]$ df -h | grep shm
tmpfs                 2.0G     0  2.0G   0% /dev/shm

[root@FWDB ~]# cat /etc/fstab | grep tmpfs
tmpfs                   /dev/shm                tmpfs   defaults,size=4G 0 0

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