About Flashback Database【每日一译】--20121117

Oracle Flashback Database,accessible from both RMAN (by means of the FLASHBACK
DATABASE command) and SQL*Plus (by means of the FLASHBACK DATABASE
statement),lets you quickly recover the entire database from logical data corruptions

or user errors.

oracle闪回数据库，可以从RMAN或者是SQLPLUS(通过FLASH DATABASE语句）访问，

让你快速还原整个数据库从逻辑数据损坏或者是用户错误中。

It is similar to conventional point in time recovery in its effects,allowing you to return
a database to its state at a time in the recent past. Flashback Database is,however,
much faster than point-in-time recovery,because it does not require restoring datafiles

from backup and it requires applying fewer changes from the archived redo logs.

它等同于传统基于时间的还原效果，允许你将数据库返回到最近的某个时间点。闪回数据库

也是这样，但它可以更快速在基于时间点的还原上，因为它不要求修复数据文件从备份中并且

它需要应用更少的改变从归档重做日志中。

Flashback Database can be used to reverse most unwanted changes to a database,as
long as the datafiles are intact. This includes returning a database to its state in

previous incarnations,that is,undoing the effects of an OPEN RESETLOGS operation.

闪回数据库可以用于返回非常多的不想的更改对于数据库来说，只要数据文件损坏，它包含

返回数据库到之前的状态，也就是，撤消了OPEN RESETLOGS的操作影响。

Flashback Database uses its own logging mechanism,creating flashback logs which
are stored in the flash recovery area. You can only use Flashback Database if flashback
logs are available. Therefore,you must set up your database in advance to create
flashback logs if you want to take advantage of this feature.

闪回数据库使用它自己的日志机制，创建的闪回日志存储在闪回区。你仅能在闪回区可用

时使用闪回数据。所以如果你想使用这个功能你必须在建立数据库之前创建闪回日志。

To enable Flashback Database,you set up a flash recovery area,and set a flashback
retention target,to specify how far back into the past you want to be able to restore
your database with Flashback Database.

开启闪回数据库功能，你创建一个闪回区，并且设置一个闪回期的时间目标，指定你想通过

闪回数据库这个功能去让数据库多久返回到你可以修复的过去的某个时间点。

From that time on,at regular intervals,the database copies images of each altered
block in every datafile into the flashback logs. These block images can later be reused
to reconstruct the datafile contents as of any moment at which logs were captured.
When a database is restored to its state at some past target time using Flashback
Database,each block changed since that time is restored from the copy of the block in
the flashback logs most immediately prior to the desired target time. The redo log is
then used to re-apply changes since the time that block was copied to the flashback
logs.

从设置闪回功能起，在一个正常的周期，数据库COPY每个改变的块的镜像在每个数据中到闪

回日志区。这些块的镜像可以稍后被重用去重构数据文件内容到任意日志被捕获的时刻。

当一个数据库被修复到过去某个目标时间点的状态通过使用闪回数据库功能 ，从那个时候每个

数据块的改变从闪回日志中拷贝的数据块中修复必须快速，根据过去设计好的目标时间。重做

日志重新应用改变从那个数据块被拷贝到闪回日志时开始。

Note: Redo logs must be available for the entire time period spanned
by the flashback logs,whether on tape or on disk. (In practice,
however,redo logs are generally needed much longer than the
flashback retention target to support point-in-time recovery.)

备注：重做日志必须是可用的在整个时间的跨越上可用的通过闪回日志，

不论是在磁带或者磁盘上。（在实际应用中，重做日志通常需要更长时间比起

闪回的支持基于时间点还原的时间目标。）