This error comes up because you tried to use the Automatic Memory Management (AMM) feature of Oracle 11g R2. Well done, but it seems that your shared memory filesystem (shmfs) is not big enough. So let’s look at the steps necessary to enlarge your shared memory filesystem to avoid the error below.
[oracle@oracle12c bin]$ ./sqlplus /nolog
SQL*Plus: Release 12.1.0.1.0 Production on Tue Aug 06 12:51:43 2013
Copyright (c) 1982, 2013, Oracle. All rights reserved.
SQL> conn / as sysdba
Connected to an idle instance.
SQL> startup
ORA-00845: MEMORY_TARGET not supported on this system
$ exit
[root@oracle12c ~]# df -h
Filesystem Size Used Avail Use% Mounted on
/dev/mapper/vg_oracle12c-lv_root
50G 18G 30G 37% /
tmpfs 1002M 480M 522M 48% /dev/shm
/dev/sda1 485M 48M 412M 11% /boot
/dev/mapper/vg_oracle12c-lv_home
39G 36G 1.2G 97% /ora
ora 173G 15G 159G 9% /media/sf_ora
p1 293G 84G 210G 29% /media/sf_p1
/dev/sr0 55M 55M 0 100% /media/VBOXADDITIONS_4.2.10_84104
[root@oracle12c ~]# mount -t tmpfs shmfs -o size=1g /dev/shm
[root@oracle12c ~]# df -h
Filesystem Size Used Avail Use% Mounted on
/dev/mapper/vg_oracle12c-lv_root
50G 18G 30G 37% /
tmpfs 1.0G 0 1.0G 0% /dev/shm
/dev/sda1 485M 48M 412M 11% /boot
/dev/mapper/vg_oracle12c-lv_home
39G 36G 1.2G 97% /ora
ora 173G 15G 159G 9% /media/sf_ora
p1 293G 84G 210G 29% /media/sf_p1
/dev/sr0 55M 55M 0 100% /media/VBOXADDITIONS_4.2.10_84104
shmfs 1.0G 0 1.0G 0% /dev/shm
shmfs 1.0G 0 1.0G 0% /dev/shm
To make the change presistent edit yout /etc/fstab file to include the option you specified.
[root@oracle12c ~]# cat /etc/fstab
[..]
tmpfs /dev/shm tmpfs size 1g 0 0
[..]
[oracle@oracle12c bin]$ ./sqlplus /nolog
SQL*Plus: Release 12.1.0.1.0 Production on Tue Aug 06 13:01:19 2013
Copyright (c) 1982, 2013, Oracle. All rights reserved.
SQL> conn / as sysdba
Connected to an idle instance.
SQL> startup
ORACLE instance started.
Total System Global Area 835104768 bytes
Fixed Size 2293880 bytes
Variable Size 562040712 bytes
Database Buffers 268435456 bytes
Redo Buffers 2334720 bytes
Database mounted.
Database opened.
SQL>