SQL> show parameter memory_target;NAME                     TYPE     VALUE
------------------------------------ ----------- ------------------------------
memory_target                 big integer 316M
SQL> show sga;Total System Global Area  330600448 bytes
Fixed Size            1336344 bytes
Variable Size          247466984 bytes
Database Buffers       75497472 bytes
Redo Buffers            6299648 bytes
SQL> alter system set memory_target=200m scope=spfile;System altered.SQL> alter system set sga_target=0 scope=spfile;System altered.SQL> alter system set pga_aggregate_target=0 scope=spfile;System altered.SQL> shutdown immediate;
Database closed.
Database dismounted.
ORACLE instance shut down.
SQL> startup
ORACLE instance started.Total System Global Area  209235968 bytes
Fixed Size            1335528 bytes
Variable Size          201330456 bytes
Database Buffers        4194304 bytes
Redo Buffers            2375680 bytes
Database mounted.
Database opened.

SQL> col ksppinm for a20;
SQL> col ksppstvl for a20;
SQL> select a.ksppinm name,b.ksppstvl value2     from x$ksppi a,x$ksppcv b3   where a.indx = b.indx4      and (a.ksppinm like '%sga_target%'5  or a.ksppinm like '%pga_aggregate_target%');NAME                                VALUE
-----------------------     -------------------
sga_target                           0
__sga_target                     142606336
pga_aggregate_target                 0
__pga_aggregate_target            67108864

[oracle@yft bin]$ oerr ora 00845
00845, 00000, "MEMORY_TARGET not supported on this system"
// *Cause: The MEMORY_TARGET parameter was not supported on this operating system or /dev/shm was not sized
// *Action: Refer to documentation for a list of supported operating systems. Or, size /dev/shm to be at leacle instance running on the system.SQL> alter system set memory_max_target=400m scope=spfile;System altered.SQL> shutdown immediate;
Database closed.
Database dismounted.
ORACLE instance shut down.
SQL> startup
ORA-00845: MEMORY_TARGET not supported on this system

[oracle@yft bin]$ df -h
Filesystem            Size  Used Avail Use% Mounted on
tmpfs                 395M     0  395M   0% /dev/shm

Mon Feb 25 12:13:21 2013
Starting ORACLE instance (normal)
WARNING: You are trying to use the MEMORY_TARGET feature. This feature requires the /dev/shm file system to be mounted for at least 419430400 bytes. /dev/shm is either not mounted or is mounted with available space less than this size. Please fix this so that MEMORY_TARGET can work as expected. Current available is 413466624 and used is 0 bytes. Ensure that the mount point is /dev/shm for this directory.
memory_target needs larger /dev/shm

[root@yft ~]# vi /etc/fstab
tmpfs                   /dev/shm                tmpfs   defaults,size=420m        0 0[root@yft ~]# mount -o remount /dev/shm
[root@yft ~]# df -h
Filesystem            Size  Used Avail Use% Mounted on
tmpfs                 420M     0  420M   0% /dev/shmSQL> startup
ORACLE instance started.Total System Global Area  418484224 bytes
Fixed Size            1336932 bytes
Variable Size          406849948 bytes
Database Buffers        4194304 bytes
Redo Buffers            6103040 bytes
Database mounted.