此时数据库是open的。

SQL> alter system set db_recovery_file_dest='/pmsdb/oracle/flash_recovery_area';

System altered.

SQL> show parameter db_r

NAME                                 TYPE        VALUE
------------------------------------ ----------- ------------------------------
db_recovery_file_dest                string      /pmsdb/oracle/flash_recovery_a
                                                 rea
db_recovery_file_dest_size           big integer 10G
db_recycle_cache_size                big integer 0
dbwr_io_slaves                       integer     0
SQL> alter system set db_recovery_file_dest_size=20m;

System altered.

SQL> show parameter db_re

NAME                                 TYPE        VALUE
------------------------------------ ----------- ------------------------------
db_recovery_file_dest                string      /pmsdb/oracle/flash_recovery_a
                                                 rea
db_recovery_file_dest_size           big integer 20M
db_recycle_cache_size                big integer 0
SQL> archive log list
Database log mode              Archive Mode
Automatic archival             Enabled
Archive destination            /pmsdb/oracle/arch
Oldest online log sequence     9
Next log sequence to archive   11
Current log sequence           11
SQL> !ls /pmsdb/oracle/arch
1_10_783515943.dbf

打算此时开启闪回功能，为了方便我直接startup mount force=shutdown abort+startup mount

SQL> startup mount force 
ORACLE instance started.

Total System Global Area 2443997184 bytes
Fixed Size                  1338224 bytes
Variable Size             754975888 bytes
Database Buffers         1677721600 bytes
Redo Buffers                9961472 bytes
Database mounted.
SQL> !ls -l /pmsdb/oracle/flash_recovery_area
total 0

SQL> alter database flashback on;
alter database flashback on
*
ERROR at line 1:
ORA-38706: Cannot turn on FLASHBACK DATABASE logging.
ORA-38714: Instance recovery required.----报错

SQL> alter database open;

Database altered.

SQL> shutdown immediate
Database closed.
Database dismounted.
ORACLE instance shut down.
SQL> startup mount
ORACLE instance started.

Total System Global Area 2443997184 bytes
Fixed Size                  1338224 bytes
Variable Size             754975888 bytes
Database Buffers         1677721600 bytes
Redo Buffers                9961472 bytes
Database mounted.
SQL>  alter database flashback on;
 alter database flashback on
*
ERROR at line 1:
ORA-38706: Cannot turn on FLASHBACK DATABASE logging.
ORA-38708: not enough space for first flashback database log file

SQL> show parameter db_r

NAME                                 TYPE        VALUE
------------------------------------ ----------- ------------------------------
db_recovery_file_dest                string      /pmsdb/oracle/flash_recovery_a
                                                 rea
db_recovery_file_dest_size           big integer 20M
db_recycle_cache_size                big integer 0
dbwr_io_slaves                       integer     0
SQL> alter system set db_recovery_file_dest_size=150m;----给的空间太少

System altered.

SQL>   alter database flashback on;
  alter database flashback on
*
ERROR at line 1:
ORA-38706: Cannot turn on FLASHBACK DATABASE logging.
ORA-38708: not enough space for first flashback database log file

SQL>  alter system set db_recovery_file_dest_size=500m;

System altered.

SQL> alter database flashback on;

Database altered.

SQL>