	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

官方解析：
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

	查看/dev/shm

[oracle@FWDB FWDB]$ df -h | grep shm

	tmpfs                 2.0G     0  2.0G   0% /dev/shm

调整/dev/shm大小

方法一：修改配置文件

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

方法二：重新挂载

mount -o remount,size=4G /dev/shm
[root@FWDB ~]# df -h | grep shm
tmpfs 4.0G 0 4.0G 0% /dev/shm
再次重启数据库，恢复正常。