oracle启动时报错ORA-00845 MEMORY_TARGET not supported on this system

oracle11g数据库在执行dbca或者调整sga后重启oracle的时候可能会出现:
ORA-00845 MEMORY_TARGET not supported on this system
究其原因就是Linux系统的shm的大小比SGA设置的小，造成的。

具体说，SGA设置4G，而shm可能只有1G，网上提供两种解决办法：
01、调整sga的大小，这个明显不是我们所希望的
02、调整shm的大小，这样相对简单，具体操作如下

vim /etc/fstab
tmpfs /dev/shm tmpfs defaults 0  0
修改为：
tmpfs /dev/shm tmpfs defaults,size=6G 0  0

重新mount下shm使其生效

mount -o remount /dev/shm

通过df可以查看下，没有问题就可以继续安装数据库或者启动数据库了！
说明：服务器上有两个不同数据库实例，启动一个实例的时候没问题。在启动第二个INSTANCE的时候报ORA-00845错误。该测试服务器仅8G内存。 tmpfs 默认是4G

4.0G  1.6G  2.4G  40% /dev/shm. 

两个实例的使用内存应该超过了4G，所以修改tmpfs，问题解决了。 
但是新问题tmpfs 大小的限制如何确定呢？观察一下内存使用在说吧

[oracle@BJRMANTS ~]$ su - root
Password:
[root@BJRMANTS ~]# vim /etc/fstab
[root@BJRMANTS ~]# df -h
Filesystem            Size  Used Avail Use% Mounted on
/dev/sda2              20G5.8G13G31%/
tmpfs                 4.0G  1.6G2.4G40%/dev/shm
/dev/sda1             485M51M409M12%/boot
/dev/sda6             197G160G28G86%/orabak
/dev/sda8              20G6.2G13G34%/oracle
/dev/sda5             690G549G106G84%/oradata
/dev/sda7             187G75G104G42%/oralog
[root@BJRMANTS ~]# mount -o remount /dev/shm
[root@BJRMANTS ~]# df -h
Filesystem            Size  Used Avail Use% Mounted on
/dev/sda2              20G5.8G13G31%/
tmpfs                 6.0G  1.6G4.5G26%/dev/shm
/dev/sda1             485M51M409M12%/boot
/dev/sda6             197G160G28G86%/orabak
/dev/sda8              20G6.2G13G34%/oracle
/dev/sda5             690G549G106G84%/oradata
/dev/sda7             187G75G104G42%/oralog
[oracle@BJRMANTS ~]$ export ORACLE_SID=oelncbi
[oracle@BJRMANTS ~]$ sqlplus / as sysdba
SQL*Plus: Release 11.2.0.3.0 Production on Wed Dec 510:48:512012
Copyright (c) 1982, 2011, Oracle.  All rights reserved.
Connected to an idle instance.
SQL> startup;
ORA-00845: MEMORY_TARGET not supported on this system
SQL>exit
Disconnected
[oracle@BJRMANTS ~]$ export ORACLE_SID=oeldzdz
[oracle@BJRMANTS ~]$ sqlplus / as sysdba
SQL*Plus: Release 11.2.0.3.0 Production on Wed Dec 510:49:332012
Copyright (c) 1982, 2011, Oracle.  All rights reserved.
Connected to:
Oracle Database 11g Enterprise Edition Release 11.2.0.3.0-64bit Production
With the Partitioning, OLAP, Data Mining and Real Application Testing options
SQL> shutdown abort;
ORACLE instance shut down.
SQL>exit
Disconnected from Oracle Database 11g Enterprise Edition Release 11.2.0.3.0-64bit Production
With the Partitioning, OLAP, Data Mining and Real Application Testing options
[oracle@BJRMANTS ~]$ export ORACLE_SID=oelncbi
[oracle@BJRMANTS ~]$ sqlplus / as sysdba
SQL*Plus: Release 11.2.0.3.0 Production on Wed Dec 510:49:512012
Copyright (c) 1982, 2011, Oracle.  All rights reserved.
Connected to an idle instance.
SQL> startup;
ORACLE instance started.
Total System Global Area  814227456 bytes
Fixed Size      2232760 bytes
Variable Size    452988488 bytes
Database Buffers   352321536 bytes
Redo Buffers      6684672 bytes
Database mounted.
Database opened.
SQL>exit
Disconnected from Oracle Database 11g Enterprise Edition Release 11.2.0.3.0-64bit Production
With the Partitioning, OLAP, Data Mining and Real Application Testing options
[oracle@BJRMANTS ~]$ export ORACLE_SID=oeldzdz
[oracle@BJRMANTS ~]$ sqlplus / as sysdba
SQL*Plus: Release 11.2.0.3.0 Production on Wed Dec 510:54:152012
Copyright (c) 1982, 2011, Oracle.  All rights reserved.
Connected to an idle instance.
SQL> startup;
ORACLE instance started.
Total System Global Area 2805977088 bytes
Fixed Size      2231672 bytes
Variable Size    603980424 bytes
Database Buffers  2181038080 bytes
Redo Buffers     18726912 bytes
Database mounted.
Database opened.