/dev/shm應該設置多大Oracle 實例啟動時才能不報ORA-00845
來源於：ORA-00845 - Which value for /dev/shm is needed to startup database without ORA-00845 (文檔 ID 1399209.1)
適用於：
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***
目標：
從Oracle database 11g開始， the Automatic Memory Management（AMM）特性需要更多的shared memory(/dev/shm)和文件描述符
Shared memory 的大小必須至少大於該台主機上每個Oracle Instance的 MEMORY_MAX_TARGET參數值和MEMORY_TARGET參數值。
如果MEMORY_MAX_TARGET參數或者MEMORY_TARGET參數被設置為了一個非零值，並且一個不正確的值被指定給了shared memory，這會導致Oracle instance啟動失敗：會報 ORA-00845 錯誤
在Linux系統中，如果操作系統系統的/dev/shm mount size 相對於Oracle SGA和PGA來說太小，你會遇到下面的錯誤：
ORA-00845: MEMORY_TARGET not supported on this system.
這個錯誤的原因是不充足的/dev/shm分配。SGA 和PGA的總內存大小，即：在初始化參數中設置的MEMORY_TARGET 或者 MEMORY_MAX_TARGET，不能大於你的操作系統上的shared memory file system (/dev/shm)大小。
細節
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
