在Oracle 10g中, Flash back家族分为以下成员: Flashback Database Flashback DropFlashback TableFlashback Query(分Flashbac
在Oracle 10g中, Flash back家族分为以下成员:
Flashback Database
Flashback Drop
Flashback Table
Flashback Query(分Flashback Query,Flashback Version Query, Flashback Transaction Query)
下面介绍一下Flashback Database
1. Flashback Database 不能解决Media Failure, 这种错误RMAN恢复仍是唯一选择
2. 如果删除了数据文件或者利用Shrink技术缩小数据文件大小,这时不能用Flashback Database技术回退到改变之前的状态,这时候就必须先利用RMAN把删除之前或者缩小之前的文件备份restore 出来, 然后利用Flashback Database 执行剩下的Flashback Datbase。
3. 如果控制文件是从备份中恢复出来的,或者是重建的控制文件,也不能使用Flashback Database。
4. 使用Flashback Database锁能恢复到的最早的SCN, 取决与Flashback Log中记录的最早SCN。
Flashback Database 架构
Flashback Database 整个架构包括一个进程Recover Writer(RVWR)后台进程,Flashback Database Log日志 和Flash Recovery Area。一旦数据库启用了Flashback Database, 则RVWR进程会启动,该进程会向Flash Recovery Area中写入Flashback Database Log, 这些日志包括的是数据块的 " 前镜像(before image)", 这也是Flashback Database 技术不完全恢复块的原因。
启用Flashback Database
数据库的Flashback Database功能缺省是关闭的,要想启用这个功能,,就需要做如下配置。
1. 配置Flash Recovery Area
要想使用Flashback Database, 必须使用Flash Recovery Area,因为Flashback Database Log只能保存在这里。 要配置的2个参数如下,一个是大小,一个是位置。如果数据库是RAC,flash recovery area 必须位于共享存储中。数据库必须处于archivelog 模式.
启用Flash Recovery Area:
SQL>ALTER SYSTEM SET DB_RECOVERY_FILE_DEST_SIZE=2G SCOPE=BOTH;
SQL>ALTER SYSTEM SET DB_RECOVERY_FILE_DEST='D:\oracle\product\10.2.0\flash_recovery_area\' SCOPE=BOTH;
禁用Flash Recovery Area:
SQL>ALTER SYSTEM SET DB_RECOVERY_FILE_DEST='' ;
对于Flash Recovery Area,Oracle 是这样建议的,flash recovery area 设置的越大,flashback database 的恢复能力就越强,因此建议flash recovery area 能够放的下所有的数据文件,增量备份,以及所有尚未备份的归档文件,当然还有它自己产生的flashback logs。
在数据库运行过程中,oracle 自动向该区域写入文件,当剩余空间不足15%的时候,它就会在alert 中增加警告,提示你空间不足。但此时不会影响数据库的正常运转,直到所有空间统统被用掉之后,oracle 首先尝试删除寻些过期的文件,冗余文件或备份过的文件,如果这些做完了,还是没有空闲空间的话,数据库就被hang 住了。
对于因Flash Recovery Area导致的数据库hang的处理,请参考:

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