闪回恢复区：存储和恢复相关的一系列备份文件的磁盘空间．优点：简化用户备份行为和备份管理．

闪回区可以存放如下文件：

控制文件；归档日志文件；闪回日志文件；控制文件和spfile自动备份文件；rman 备份集和拷贝数据文件．

闪回区的缺点：当闪回区的空间耗尽的时候，数据库将陷入停顿状态．

如何预防：当闪回区达到空间的８５％时，alert文件会发生告警．当达到９７％时，会写入严重警告信息．

测试：　

SQL> show parameter recovery

NAME                                 TYPE        VALUE
------------------------------------ ----------- ------------------------------
db_recovery_file_dest                string      /oracle/flash_recovery_area
db_recovery_file_dest_size           big integer 2G
recovery_parallelism                 integer     0

闪回区的大小是由＂db_recovery_file_dest_size  ＂决定．

闪回区的位置由＂db_recovery_file_dest＂决定．

RECOVERY_PARALLELISM specifies the number of processes to participate in instance or crash recovery. A value of 0 or 1 indicates that recovery is to be performed serially by one process.(文档给于的解释）．

SQL> alter system set db_recovery_file_dest_size=5G scope=both;

System altered.

SQL> show parameter recovery

NAME                                 TYPE        VALUE
------------------------------------ ----------- ------------------------------
db_recovery_file_dest                string      /oracle/flash_recovery_area
db_recovery_file_dest_size           big integer 5G
recovery_parallelism                 integer     0

经过一次rman备份后，查看flashcover空间剩余容量．

SQL> select substr(name,1,30)name,space_limit as quota,
  2  space_used as used,
  3  space_reclaimable as reclaimable,
  4  number_of_files as files
  5  from v$recovery_file_dest;

NAME                                QUOTA       USED RECLAIMABLE      FILES
------------------------------ ---------- ---------- ----------- ----------
/oracle/flash_recovery_area    5368709120  628910592    20490752          3

说明闪回区的空间只能增大不能在缩小了．

SQL> alter system set db_recovery_file_dest_size=1.5G scope=both;
alter system set db_recovery_file_dest_size=1.5G scope=both
*
ERROR at line 1:
ORA-32005: error while parsing size specification [1.5G]

其次，9I的log_archive_start已经不能再10G中使用啦

SQL> show parameter log_archive_start

NAME                                 TYPE        VALUE
------------------------------------ ----------- ------------------------------
log_archive_start                    boolean     FALSE