I recently helped to create a lab dedicated to a training about Oracle backup and recovery, with Oracle 11g databases. While creating several databases on an Oracle Enterprise Linux 6.1, I wanted to resize the tmpfs to allocate more memory to the databases for AMM (Automatic Memory Management). That was a small challenge ūüôā …

By default, the value of the tmpfs is the half of the installed memory. So, on my 3 GB memory sized environment, the tmpfs was sized to 1,5 GB per default. This was not enough to run my 3 databases concurrently. In this case, resize the tmpfs was necessary.

The common way to resize the tmpfs is to edit the fstab file located in the /etc directory, and to add the “size=” attribute for the tmpfs row. Alter this file is supposed to allow the new size to be static, meaning that the tmpfs will keep the new value at each startup.

In the following example I tryed to resize the tmpfs to 2 GB:

tmpfs         /dev/shm       tmpfs   defaults,size=2g    0 0

But surprise, after saving the file and restarting the machine, tmpfs was still 1,5 GB sized:

[[email protected] ~]# df -h
Filesystem            Size  Used Avail Use% Mounted on
tmpfs                 1.5G  420M  1.1G  28% /dev/shm


However, if I hot resize the tmpfs with the following command, the size is updated successfully:

mount -o remount /dev/shm

[[email protected] ~]# df -h
Filesystem            Size  Used Avail Use% Mounted on
tmpfs                 2.0G  420M  1.6G  21% /dev/shm


In fact, from Oracle Enterprise Linux 6.0, this parameter is not persistent: at startup, the tmpfs value is set to its default value, regardless what is recorded in the fstab. This problem is also issued on Red Hat 6, and it seems to be a bug proper to this Linux distribution.

To fix this problem, a simple way is to automatically run the previous command mount -o remount /dev/shm at startup. Tmpfs must be resized before starting Oracle to avoid any risk of database shutdown. Two cases:

  • Database is not set to start automatically at server startup: the script /etc/rc.d/rc.local can be used. This script allows to execute commands or other scripts. It is executed after all other init scripts, but if Oracle is started manually it is not a problem.
  • Database is set to start automatically at server startup (using for example the dbi services DMK service_start_stop script): it is better to create a specific script to run the command, and to configure it with chkconfig to execute before the oracle services start script, at wanted run levels.


Example of using rc.local script

Some people prefer to use another way to fix the problem, modifying the /etc/rc.d/rc.sysinit file. This technique is not developped here, but just know that the sysinit file is responsible of few operations such as mounting filesystems or setup networking, and that it is the first script run by the /etc/inittab file, at /sbin/init process startup.

Alter this file is at your own risks, it is not advised to change it.

To conclude with something interesting: I tried to relocate the tmpfs from /dev/shm to /mnt/shm (for example). This time, the size parameter was loaded successfully at startup. But I quickly realized that after starting an instance, the new path of tmpfs remained empty and unused.

What is strange is that the instance was running correctly with AMM, which mode is supposed to require the shared memory file system /dev/shm (tmpfs):

SQL> show parameter memory
NAME                                 TYPE        VALUE
------------------------------------ ----------- -----------
memory_max_target                    big integer 800M
memory_target                        big integer 800M
[[email protected] ~]# df -h
Filesystem            Size  Used Avail Use% Mounted on
tmpfs                 2.0G     0  2.0G   0% /mnt/shm


So if the instance is running, and no tmpfs appears to be used, where can be loaded the SGA ? The response is in /dev/shm ! Yes, no matter if a tmpfs filesystem is declared or not in the fstab, Oracle still uses the /dev/shm path for SGA with AMM.

[[email protected] ~]# ls -altr /dev/shm
total 490836
-rw-r-----. 1 oracle oinstall 4194304 Oct 31 13:45 ora_DBTEST1_425984_0
-rw-r-----. 1 oracle oinstall 4194304 Oct 31 13:45 ora_DBTEST1_425984_1
-rw-r-----. 1 oracle oinstall       0 Oct 31 13:45 ora_DBTEST1_458753_0

And tmpfs still gets the default size value:

[[email protected] ~]# df -h /dev/shm
Filesystem            Size  Used Avail Use% Mounted on
-                     1.5G  480M  1.1G  32% /dev/shm