linux ipcs确定共享内存使用情况

环境：

[oracle@simpleit ~]$ 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@simpleit ~]$ 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@simpleit ~]$ ipcs -b
ipcs: invalid option -- b
usage : ipcs -asmq -tclup
        ipcs [-s -m -q] -i id
        ipcs -h for help.
[oracle@simpleit ~]$ ipcs

------ Shared Memory Segments --------
key        shmid      owner      perms      bytes      nattch     status
0xe9b42ad8 2228225    oracle    640        88080384   13
0x8cdf18fc 2359299    oracle    640        528482304 26

------ Semaphore Arrays --------
key        semid      owner      perms      nsems
0xbbe791cc 98304      oracle    640        44
0x9d085d3c 491521     oracle    640        154

------ Message Queues --------
key        msqid      owner      perms      used-bytes   messages
[oracle@simpleit ~]$ ipcs -m

------ Shared Memory Segments --------
key        shmid      owner      perms      bytes      nattch     status
0xe9b42ad8 2228225    oracle    640        88080384   13
0x8cdf18fc 2359299    oracle    640        528482304 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

一个实例：
两条Linux命令彻底杀死Oracle http://www.zzzyk.com/database/201201/116094.html

摘自 Laughing

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