shmfs /dev/shm tmpfs size=2G  0 

oratest11:~ # lsof /dev/shm/
oratest11:~ #

oratest11:/dev/shm/sysconfig # ls -l
total  40 
-rw-r--r-- 1 root root 25 Dec  7 15:24 config-eth3
-rw-r--r-- 1 root root  3 Dec  7 15:24 config-lo
-rw-r--r-- 1 root root 61 Dec  7 15:24 if-eth3
-rw-r--r-- 1 root root 37 Dec  7 15:24 if-lo
-rw-r--r-- 1 root root  7 Dec  7 15:24 ifup-eth3
-rw-r--r-- 1 root root  7 Dec  7 15:24 ifup-lo
-rw-r--r-- 1 root root  3 Dec  7 15:24 network
-rw-r--r-- 1 root root  8 Dec  7 15:24 new-stamp-1
-rw-r--r-- 1 root root  8 Dec  7 15:24 new-stamp-2
-rw-r--r-- 1 root root  8 Dec  7 15:24 new-stamp-3
-rw-r--r-- 1 root root  0 Dec  7 15:24 ready-eth3
-rw-r--r-- 1 root root  0 Dec  7 15:24 ready-lo
-rw-r--r-- 1 root root  0 Dec  7 15:24 ready-sit0
drwxr-xr-x  2  root root  60  Dec   7   15 : 24  tmp
oratest11:/dev/shm/sysconfig #

echo "Mount shmfs ..."
mount -t tmpfs shmfs -o size=2g /dev/shm

echo "Umount shmfs ..."
umount /dev/shm

shmfs                  2 .0G   1 .2G  835M   60 % /dev/shm

oratest11:/dev/shm # ls -l
total  1242728 
-rw-r----- 1 oracle oinstall 16777216 Dec  7 20:32 ora_oratest11_65536_0
-rw-r----- 1 oracle oinstall        0 Dec  7 20:27 ora_oratest11_65536_1
-rw-r----- 1 oracle oinstall        0 Dec  7 20:27 ora_oratest11_65536_10
....

oratest11:/dev/shm # ls -l | grep sys
drwxr-xr-x  3  root   root           240  Dec   7   20 : 28  sysconfig
oratest11:/dev/shm #

oratest11:/dev/shm/sysconfig # ls -l
total  28 
-rw-r--r-- 1 root root 25 Dec  7 20:28 config-eth3
-rw-r--r-- 1 root root  3 Dec  7 20:28 config-lo
-rw-r--r-- 1 root root 61 Dec  7 20:28 if-eth3
-rw-r--r-- 1 root root 37 Dec  7 20:28 if-lo
-rw-r--r-- 1 root root  7 Dec  7 20:28 ifup-eth3
-rw-r--r-- 1 root root  7 Dec  7 20:28 ifup-lo
-rw-r--r-- 1 root root  4 Dec  7 20:28 network
-rw-r--r-- 1 root root  0 Dec  7 20:28 ready-eth3
-rw-r--r-- 1 root root  0 Dec  7 20:28 ready-lo
drwxr-xr-x  2  root root  60  Dec   7   20 : 28  tmp
oratest11:/dev/shm/sysconfig #

Symptoms

On a Linux system, trying to start up an 11g instance could fail with the following error:

ORA- 845 : MEMORY_TARGET not supported on this system
...

Solution
 1 . If you are installing Oracle 11g on a Linux system, note that Memory Size (SGA and PGA), which sets 
the initialization parameter MEMORY_TARGET or MEMORY_MAX_TARGET, cannot be greater than the shared memory filesystem (/dev/shm) on your operating system. To resolve the current error, increase the /dev/shm file size. For example:

# mount -t tmpfs shmfs -o size=7g /dev/shm

Also, to make this change persistent across system restarts, add an entry in /etc/fstab similar to the following:

shmfs /dev/shm tmpfs size=7g  0 

 2 . This error may also occur if /dev/shm is not properly mounted. Make sure your df output is similar to the following:

$ df -k
Filesystem 1K-blocks Used Available Use% Mounted on
...
shmfs  6291456   832356   5459100   14 % /dev/shm


 3 . If configuring AMM is not possible due to lack of space on /dev/shm mount point, you can configure ASMM instead of AMM, i.e. set SGA_TARGET, SGA_MAX_SIZE and PGA_AGGREGATE_TARGET instead of MEMORY_TARGET.