deferred指的是当前会话不生效，以后的会话生效。
SQL> select name from v$parameter where issys_modifiable='DEFERRED';

NAME
--------------------------------------------------------------------------------
backup_tape_io_slaves
recyclebin
audit_file_dest
object_cache_optimal_size
object_cache_max_size_percent
sort_area_size
sort_area_retained_size
olap_page_pool_size

8 rows selected.

SQL> show parameter audit

NAME                                 TYPE        VALUE
------------------------------------ ----------- ------------------------------
audit_file_dest                      string      /u01/app/oracle/admin/test/adu
                                                 mp
audit_sys_operations                 boolean     FALSE
audit_syslog_level                   string
audit_trail                          string      DB
修改失败
SQL> alter system set audit_file_dest='/tmp';
alter system set audit_file_dest='/tmp'
                                      *
ERROR at line 1:
ORA-02096: specified initialization parameter is not modifiable with this
option

修改成功
SQL> alter system set audit_file_dest='/tmp' deferred;

System altered.

SQL>
SQL>
SQL> show parameter audit

NAME                                 TYPE        VALUE
------------------------------------ ----------- ------------------------------
audit_file_dest                      string      /u01/app/oracle/admin/test/adu
                                                 mp
audit_sys_operations                 boolean     FALSE
audit_syslog_level                   string
audit_trail                          string      DB
SQL> quit
Disconnected from Oracle Database 11g Enterprise Edition Release 11.2.0.1.0 - Production
With the Partitioning, OLAP, Data Mining and Real Application Testing options
[oracle@node1 ~]$ sqlplus / as sysdba

SQL*Plus: Release 11.2.0.1.0 Production on Sat May 3 12:26:01 2014

Connected to:
Oracle Database 11g Enterprise Edition Release 11.2.0.1.0 - Production
With the Partitioning, OLAP, Data Mining and Real Application Testing options

SQL> show parameter audit

NAME                                 TYPE        VALUE
------------------------------------ ----------- ------------------------------
audit_file_dest                      string      /tmp
audit_sys_operations                 boolean     FALSE
audit_syslog_level                   string
audit_trail                          string      DB

修改参数必须加deferred的参数