转自：http://www.cnblogs.com/killkill/archive/2010/09/10/1823690.html，作者：killkill

ORA-00845: MEMORY_TARGET not supported on this system

      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 shmtmpfs                 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：
[root@FWDB ~]# cat /etc/fstab | grep tmpfstmpfs                  /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：  
[root@FWDB ~]# mount -o remount,size=4G /dev/shm[root@FWDB ~]# df -h | grep shmtmpfs                 4.0G     0  4.0G   0% /dev/shm    再次启动数据库，没有报错了。