ipcs

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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。

[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

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