ORA-00845: MEMORY_TARGET not supported on this system - automatic memory management

Oracle error "ORA-00845: MEMORY_TARGET not supported on this system" is encountered in oracle 11g and 12c databases when instance tries to use automatic memory management and tmpfs mount point is less than the value specified in oracle database instance parameter memory_max_target.  Remedy is unmount the tmpfs and mount with increased space for tmps but also recommended is same change in /etc/fstab for permanent effect to take place.

SQL> alter system set memory_max_target=1024m scope=spfile;
System altered.
SQL> startup force
ORA-00845: MEMORY_TARGET not supported on this systemSQL> exit
Disconnected from Oracle Database 12c Enterprise Edition Release 12.1.0.1.0 - 64bit Production
With the Partitioning, OLAP, Advanced Analytics and Real Application Testing options
[oracle@orcl12c ~]$ df -h
Filesystem            Size  Used Avail Use% Mounted on
/dev/sda5              13G   10G  2.2G  83% /
tmpfs                 939M   72K  939M   1% /dev/shm/dev/sda1              73M   29M   41M  41% /boot
/dev/sda2            1008M   43M  915M   5% /tmp
vmshare               1.7T  161G  1.5T  10% /media/sf_vmshare
[oracle@orcl12c ~]$ su -
Password:
[root@orcl12c ~]# umount tmpfs
umount: /dev/shm: device is busy.
        (In some cases useful info about processes that use
         the device is found by lsof(8) or fuser(1))
[root@orcl12c ~]# umount -l tmpfs
[root@orcl12c ~]# mount -t tmpfs  tmpfs -o size=10240m /dev/shm
[root@orcl12c ~]# df -h
Filesystem            Size  Used Avail Use% Mounted on
/dev/sda5              13G   10G  2.2G  83% /
/dev/sda1              73M   29M   41M  41% /boot
/dev/sda2            1008M   43M  915M   5% /tmp
vmshare               1.7T  161G  1.5T  10% /media/sf_vmshare
tmpfs                  10G     0   10G   0% /dev/shm
[root@orcl12c ~]# cat /etc/fstab
.
.
tmpfs                   /dev/shm                tmpfs   defaults        0 0
.
.

[root@orcl12c ~]# vi /etc/fstab
[root@orcl12c ~]# cat /etc/fstab
.
.
#tmpfs                   /dev/shm                tmpfs   defaults        0 0
tmpfs                   /dev/shm                tmpfs   size=1024m      0 0
.
.
[root@orcl12c ~]# exit
logout
[oracle@orcl12c ~]$ sqlplus / as sysdba
SQL*Plus: Release 12.1.0.1.0 Production on Sun Aug 4 15:35:40 2013
Copyright (c) 1982, 2013, Oracle.  All rights reserved.
Connected to an idle instance.
SQL> startup
ORACLE instance started.
Total System Global Area 1068937216 bytes
Fixed Size                  2296576 bytes
Variable Size            1010828544 bytes
Database Buffers           50331648 bytes
Redo Buffers                5480448 bytes
Database mounted.
Database opened.
SQL> show parameter memory_max_target
NAME                                 TYPE        VALUE
------------------------------------ ----------- ------------------------------
memory_max_target                    big integer 1G
SQL> show parameter sga_max_size
NAME                                 TYPE        VALUE
------------------------------------ ----------- ------------------------------
sga_max_size                         big integer 1G
SQL> rem sga_max_size is set automatically equal to the memory_max_target
SQL> show parameter memory_target
NAME                                 TYPE        VALUE
------------------------------------ ----------- ------------------------------
memory_target                        big integer 0
SQL> alter system set memory_target=1024m;
System altered.
SQL>REM now automatic memory management has been configured