【翻译自mos文章】/dev/shm应该设置多大Oracle 实例启动时才能不报ORA-00845

/dev/shm应该设置多大Oracle 实例启动时才能不报ORA-00845
来源于：ORA-00845 - Which value for /dev/shm is needed to startup database without ORA-00845 (文档 ID 1399209.1)

适用于：
Oracle Database - Standard Edition - Version 11.1.0.6 and later
Information in this document applies to any platform.
***Checked for relevance on 17-Jul-2015***

目标：
从Oracle database 11g开始， the Automatic Memory Management（AMM）特性需要更多的shared memory(/dev/shm)和文件描述符
Shared memory 的大小必须至少大于该台主机上每个Oracle Instance的 MEMORY_MAX_TARGET参数值和MEMORY_TARGET参数值。
如果MEMORY_MAX_TARGET参数或者MEMORY_TARGET参数被设置为了一个非零值，并且一个不正确的值被指定给了shared memory，这会导致Oracle instance启动失败：会报 ORA-00845 错误

在Linux系统中，如果操作系统系统的/dev/shm mount size 相对于Oracle SGA和PGA来说太小，你会遇到下面的错误：

ORA-00845: MEMORY_TARGET not supported on this system.

这个错误的原因是不充足的/dev/shm分配。SGA 和PGA的总内存大小，即：在初始化参数中设置的MEMORY_TARGET 或者 MEMORY_MAX_TARGET，不能大于你的操作系统上的shared memory file system (/dev/shm)大小。

细节

The alert log will provide the expected value for /dev/shm.

WARNING: You are trying to use the MEMORY_TARGET feature. 
This feature requires the /dev/shm file system to be mounted for at least 1073741824 bytes. 
/dev/shm is either not mounted or is mounted with available space less than this size. 
Please fix this so that MEMORY_TARGET can work as expected. 
Current available is 1049145344 and used is 430080 bytes. 
Ensure that the mount point is /dev/shm for this directory. memory_target needs larger /dev/shm

Here the alert log confirms:

current size : 1049145344 bytes
expected size: 1073741824 bytes (at least)

The current size of /dev/shm is also confirmed by the 'df' command -- Current value

# df -k /dev/shm
Filesystem 1K-blocks Used Available Use% Mounted on
tmpfs 1024976 420 1024556 1% /dev/shm

To fix the current issue the solution would be to allocate at least 1073741824 bytes for /dev/shm.

# mount -t tmpfs shmfs -o size=1073741824 /dev/shm 
# df -k /dev/shm
Filesystem 1K-blocks Used Available Use% Mounted on
shmfs 1048576 0 1048576 0% /dev/shm

To retain the value across machine startups, make sure that the /etc/fstab mount database contains a reference to this new value, like in:

shmfs /dev/shm tmpfs size=1073741824 0