Swap

First of all, login as root and have a look at the filesystem:

[root@oracle-em oracle]# df -h
Filesystem            Size  Used Avail Use% Mounted on
/dev/mapper/vg_oracleem-lv_root
                       93G   19G   69G  22% /
tmpfs                 5.9G  112K  5.9G   1% /dev/shm
/dev/sda1             485M   99M  362M  22% /boot

So we can see that tmpfs has a size of 6GB. We can change the size of that filesystem by issuing the following command (where “12g” is the size I want for my MEMORY_TARGET):

[root@oracle-em oracle]# mount -t tmpfs shmfs -o size=12g /dev/shm

This command (re)mounts the shmfs filesystem (check this post for more information about shmfs) with the option “size=12g“.
The shared memory file system should be big enough to accommodate the MEMORY_TARGET and MEMORY_MAX_TARGET values, or Oracle will throw the ORA-00845 error. Note that when changing something with the mount command, the changes are not permanent.
To make the change persistent, edit your /etc/fstab file to include the option you specified above:

[root@oracle-em ~]# cat /etc/fstab
[..]
tmpfs                   /dev/shm                tmpfs   size=12g        0 0