环境：
[Oracle@ www ~]$ uname -a
Linux simpleit.domain.cn 2.6.18-194.el5 #1 SMP Fri Apr 2 14:58:35 EDT 2010 i686 athlon i386 GNU/Linux
[oracle@ www ~]$ cat /etc/RedHat-release
CentOS release 5.5 (Final)
ipcs确定共享内存的使用情况，该命令用于输出系统中处于active状态的消息队列，共享内存段以及信号量的有关消息。
DESCRIPTION
The ipcs utility shall write information about active interprocess communication facili-ties.
Without options, information shall be written in short format for message queues, shared
memory segments, and semaphore sets that are currently active in the system. Otherwise, the
information that is displayed is controlled by the options specified.
-b Write information on maximum allowable size. (Maximum number of bytes in messages on
queue for message queues, size of segments for shared memory, and number of
semaphores in each set for semaphores.)
[oracle@ www ~]$ ipcs -b
ipcs: invalid option -- b
usage : ipcs -asmq -tclup
ipcs [-s -m -q] -i id
ipcs -h for help.
[oracle@ www ~]$ ipcs
------ Shared Memory Segments --------
keyshmid owner perms bytes nattch status
0xe9b42ad8 2228225oracle64088080384 13
0x8cdf18fc 2359299oracle640528482304 26
------ Semaphore Arrays --------
keysemid owner perms nsems
0xbbe791cc 98304 oracle64044
0x9d085d3c 491521 oracle640154
------ Message Queues --------
keymsqid owner perms used-bytes messages
[oracle@ www~]$ ipcs -m
------ Shared Memory Segments --------
keyshmid owner perms bytes nattch status
0xe9b42ad8 2228225oracle64088080384 13
0x8cdf18fc 2359299oracle640528482304 26
通常最好让整个SGA处于一个单一的共享内存段，因为跟踪一个以上的段将需要额外的开销，在这些段来回切换也需要时间。可以增加/etc/sysctl.conf文件kernel.shmmax参数值，以增加一个单一的共享内存段的最大尺寸。
# Controls the maximum shared segment size, in bytes
kernel.shmmax = 4294967295
使用ipcs可以查看系统的SGA是否由多个不连续的内存块组成，如果数据库失败，在释放内存时就会出现问题。使用ipcrm命令（仅仅用于在数据库失败之后SGA碎片没有被释放的情况）可以从内存中去除SGA碎片。不要在一个正在运行的数据库中运行ipcrm命令。
ipcrm命令
DESCRIPTION
The ipcrm utility shall remove zero or more message queues, semaphore sets, or shared mem-
ory segments. The interprocess communication facilities to be removed are specified by the
options.
Only a user with appropriate privilege shall be allowed to remove an interprocess communi-
cation facility that was not created by or owned by the user invoking ipcrm.
-q msgid --队列id
Remove the message queue identifier msgid from the system and destroy the message
queue and data structure associated with it.
-m shmid --共享内存id
Remove the shared memory identifier shmid from the system. The shared memory segment
and data structure associated with it shall be destroyed after the last detach.
-s semid --信号量id
Remove the semaphore identifier semid from the system and destroy the set of
semaphores and data structure associated with it.
比如 ipcrm -m 2359299