这个问题是在一次测试中发现的。在虚拟机上恢复生产库的备份，服务器是24G内存，而虚拟机才分配了2G。好了，言归正传：
当系统报出ORA-00845错误时，是由于Oracle 11g的内存管理新特性要求更多的共享内存和文件描述符导致的。解决方法呢就是更改/dev/shm的大小，当然也可以更改MEMORY_MAX_TARGET大小，但是这个方法不好。
下面是官方手册里的说明，为了让暂时看英文还头大的朋友稍微舒服点儿，在每段后面都加上中文的简要说明。
Automatic Memory Management
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 must be at least the greater of the MEMORY_MAX_TARGET and MEMORY_TARGET parameters for each Oracle instance on the computer. If the MEMORY_MAX_TARGET parameter or the MEMORY_TARGET parameter is set to a nonzero value, and an incorrect size is assigned to the shared memory, it results in an ORA-00845 error at startup. On Linux systems, if the operating system /dev/shm mount size is too small for the Oracle system global area (SGA) and program global area (PGA), it results in an ORA-00845 error.
从11g开始，AMM特性呢要求更多的共享内存和文件描述符。对于计算机上的每个oracle实例，它的共享内存大小必须至少比MEMORY_MAX_TARGET和MAX_TARGET都大。如果你启用了AMM，也就是上面的两个参数设置成了非零值，并且设置了不正确的共享内存值，那么就会在启动时出现ORA-00845的错误。
The number of file descriptors for each Oracle instance must be at least 512*PROCESSES. The limit of descriptors for each process must be at least 512. If file descriptors are not sized correctly, you see an ORA-27123 error from various Oracle processes and potentially Linux Error EMFILE (Too many open files)in non-Oracle processes.
这一段是说ORA-27123错误的原因，是文件描述符没有达到oracle要求的最小值。
To determine the amount of shared memory available, enter the following command:
执行下面的命令，查看系统可用共享内存值大小
# df -h /dev/shm/
Note:
The MEMORY_MAX_TARGET and MEMORY_TARGET parameters cannot be used when the LOCK_SGA parameter is enabled, or with HugePages on Linux.
On the Initialization Parameters page, note the Memory Size (SGA and PGA), which sets the initialization parameter MEMORY_TARGET or MEMORY_MAX_TARGET. The initialization parameters cannot be greater than the shared memory file system on the operating system. For example, if the shared memory file system allocation on your system is 1 GB, but you set Memory Size (MEMORY_TARGET) to 2 GB, then the following error messages are displayed during database startup:
ORA-00845: MEMORY_TARGET not supported on this system
ORA-01078: Failure in processing system parameters
这一段就是说了一个具体的例子。在参数文件中，那两个初始化参数不能比操作系统上的共享内存文件系统大。例如，你的系统上共享内存文件系统分配的值是1GB，但是你设置的MEMORY_TARGET到2GB了，那么00845就来了。
In addition, if you click All Initialization Parameters and the global database name is longer than eight characters, then the database name value (in the DB_NAME parameter) is truncated to the first eight characters, and the DB_UNIQUE_NAME parameter value is set to the global name.
这个不是本篇文章要说的，略过~~~
The workaround, if you encounter the ORA-00845 error, is to increase the /dev/shm mountpoint size.
For example:
# mount -t tmpfs shmfs -o size=7g /dev/shm
To make this change persistent across system restarts, add an entry in /etc/fstab similar to the following:
shmfs /dev/shm tmpfs size=7g 0
上面的这一串就是告诉咱们如何增加/dev/shm的大小，并且如何永久生效