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=== Init file or a pfile ===
A PFILE is a traditional text based init.ora parameter file. Typically this resides on the server in the $ORACLE_BASE/admin/SID/pfile directory, with a symbolic link pointing to it from the $ORACLE_HOME/dbs directory. In addition, a DBA may keep copies of this file on their local PC to allow remote startup:

%%
SQL> CONNECT sys/password AS SYSDBA
SQL> STARTUP PFILE=C:\init_instance_name.ora
%%

A PFILE is necessary in order to create a SPFILE to enable persistent initialization parameters.
If you already have a SPFILE, a PFILE can be generated from it using one of the following:

%%
CREATE PFILE FROM SPFILE;
CREATE PFILE FROM SPFILE = 'production.ora';
CREATE PFILE = '$ORACLE_HOME/dbs/init_instance.ora' FROM SPFILE;
CREATE PFILE = '$ORACLE_HOME/dbs/init_instance.ora' FROM SPFILE = '$ORACLE_HOME/dbs/init_instance.ora';
%%

A SPFILE is created from a PFILE using one of the following commands:

%%
SQL> CREATE SPFILE FROM PFILE = '$ORACLE_HOME/dbs/init_instance.ora';
SQL> CREATE SPFILE = '$ORACLE_HOME/dbs/my_spfile.ora' FROM PFILE = '$ORACLE_HOME/dbs/init_instance.ora';
%%

== pfile or spfile? ==
%%
SELECT DECODE(value, NULL, 'PFILE', 'SPFILE') "Init File Type"
FROM sys.v_$parameter WHERE name = 'spfile';
%%

==Change SGA==
%%
SYS@etest> startup nomount
ORACLE instance started.

Total System Global Area 264241152 bytes
Fixed Size 1301408 bytes
Variable Size 216540256 bytes
Database Buffers 46137344 bytes
Redo Buffers 262144 bytes


SYS@etest> alter system set sga_target=250M scope=both;
System altered.

SYS@etest> alter database mount;
Database altered.

SYS@etest> alter database open;
Database altered.
%%

In 11g,memory are managed by the ** memory_target and memory_max_target** parameters.