Oracle 闪回区(Oracle Flash recovery area)
Oracle闪回区已经有了好几载的岁月了，在10g的时候就被推出一直延续到现在。Oracle闪回区是为RMAN准备的，尽管抛开它我们可以同样
Oracle闪回区已经有了好几载的岁月了，在10g的时候就被推出一直延续到现在。Oracle闪回区是为RMAN准备的，尽管抛开它我们可以同样完成数据库备份恢复的相关工作，Oracle依旧强烈推荐使用且它让其成为RMAN的组件之一。这是因为该区域可集中简化管理所有和备份恢复工作，这也是其魅力所在。本文描述了Oracle闪回区的特性及其配置闪回区，监控闪回区等。
1、闪回区特性及其配置
闪回区
就是分配一个特定的目录位置(普通磁盘上的目录或ASM磁盘)来存放一些特定的恢复文件，用于集中和简化管理数据库恢复工作。
通过配置闪回区，可以避免在多实例归档到相同磁盘时，磁盘空间耗尽而影响所有实例的情行。也就是说缩小影响范围。
闪回区单词缩写为FRA，下面的描述FRA即指的是闪回区，也可能存在混用闪回区或FRA的情形。
闪回区可存储文件的类型
完全的数据文件备份
增量备份
数据文件副本
当前的控制文件，备份的控制文件，spfile文件，快照控制文件
联机日志文件，归档日志
块跟踪文件、闪回日志
启用与禁用闪回区
设置下面的初始化参数(spfile或pfile文件)
DB_RECOVERY_FILE_DEST_SIZE：用于设置闪回区的大小，此参数应优先于DB_RECOVERY_FILE_DEST参数修改，否则ORA-32001
DB_RECOVERY_FILE_DEST：用于设置闪回区的目录
DB_FLASHBACK_RETENTION_TARGET：指定数据库可以闪回的时间范围，单位为分钟，默认1440分钟，也就是一天
数据库闪回分为多种情形，其闪回日志存放在FRA，可闪回的时间同时还取决于闪回恢复区的大小
将DB_RECOVERY_FILE_DEST参数设置为空，可以停用FRA，但是启用flashback database，则不能取消FRA，需要先禁用flashback database
数据库FRA可以为每个数据库配置不同的闪回区，也可以为多个数据库配置相同路径的闪回区
当为多个数据库配置相同的闪回区位置时，应考虑闪回区所在磁盘挂载点的总大小以及使用不同的DB_NAME
多个数据库或实例使用相同的闪回区时，其路径为：FRA_HOME/
FRA中的保留策略
FRA中的文件的保留与否由RMAN保留策略来决定。通过执行RMAN configure retention policy命令来设置其策略
对于没有关联保留策略或是永久文件，文件永远不会被删除
对于关联保留策略的文件，没有过时的情况下不会被删除，一旦过时，在空间压力下回自动被删除
FRA与多路日志归档
如果设置了归档日志参数log_archive_dest_n，归当时会使用该位置而不是使用FRA
如果未设置参数log_archive_dest_n，而是启用FRA，则可以不需要再单独设置归档日志参数log_archive_dest_n，归档日志会位于FRA
对于启用FRA后，不可再设置log_archive_dest、log_archive_duplex_dest，也就是说存在排他模式。可参考:Oracle 归档日志
对于既要归档到log_archive_dest_n，又要归档到FRA的情形，需要设置参数log_archive_dest_10(缺省情况)给FRA，如下：
alter system set log_archive_dest_10='LOCATION=USE_DB_RECOVERY_FILE_DEST';
alter system set log_archive_dest_1='LOCATION=/u03/database/archdir';
2、FRA空间分配与监控
对于启用FRA特性之后，FRA帮助管理全部的磁盘空间分配。Oracle会自动监控FRA空间的使用情况。
关于FRA空间分配大小应考虑当前数据库备份的方式，备份是否压缩，每天归档日志大小，是否启用了闪回等多项因素综合来考量
FRA中可用空间达到不安全的程度或不够用的情形，可以通过OEM或DBA_OUTSTANDING_ALERTS获得相关信息
会在Alert日志文件中生成警告
当可回收空间低于DB_RECOVERY_FILE_DEST_SIZE定义值的15%时，生成警报
当可回收空间低于DB_RECOVERY_FILE_DEST_SIZE定义值的3%时，生成严重警报
当整个FRA被完全填满时，系统不可用，直接给你ORA-19815，ORA-19809 ：limit exceeded for recovery files
注FRA空间被填满，不表示当前的磁盘挂载点空间不够。
FRA空间不够用或出现严重告警的情形，应考虑从以下方面着手解决
如果仅仅是参数DB_RECOVERY_FILE_DEST_SIZE大小限制，磁盘空闲空间很多，，则直接修改该参数到一个更大的值
如果磁盘空闲空间不多，应考虑分配更多的磁盘空间给文件系统，然后再修改DB_RECOVERY_FILE_DEST_SIZE参数到一个更大的值
如果无法分配额外的磁盘空间，可以考虑迁移FRA到有较多可用空间的另外一个文件系统
可以使用backup recovery area命令将整个FRA内容移动到另外的位置
删除FRA中较早备份集或归档日志，建议使用RMAN命令来删除，若直接从os删除，Oracle认为FRA的空间并没有释放(需要crosscheck再delete)
--Author : Leshami
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