ORA-00845: MEMORY_TARGET not supported on this system

SQL> startup nomount;
ORA-00845: MEMORY_TARGET not supported on this system

This error comes up because we tried to use the Automatic Memory Management (AMM) feature of Oracle 11g R2. But it seems that your shared memory filesystem (shmfs) is not big enough.

[oracle@oel6 ~]$ df -h
Filesystem            Size  Used Avail Use% Mounted on
/dev/mapper/vg_oel6-lv_root
                       43G   15G   26G  37% /
tmpfs                1001M  212M  789M  22% /dev/shm
/dev/sda1             485M   55M  405M  12% /boot
.host:/               245G  126G  119G  52% /mnt/hgfs

We can change the size of that filesystem by issuing the following command:

mount -t tmpfs shmfs -o size=2g /dev/shm

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

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

Now go and enjoy your automatically managed memory configuration!