Control Files Undo Segments【每日一译】--2012-11-06

The control file contains the record of the physical structures of the database and their

status. Several types of information stored in the control file are related to backup and

recovery:

控制文件包含了数据库物理的结构记录和它们的状态。几种类型的信息存储在控制文件中，它们与

备份和还原是相关的。

■ Database information (RESETLOGS SCN and time stamp)

■ Tablespace and datafile records (filenames, datafile checkpoints, read/write status,

offline ranges)

■ Information about redo threads (current online redo log)

■ Log records (log sequence numbers, SCN range in each log)

■ A record of past RMAN backups

■ Information about corrupt datafile blocks

#数据库信息（RESETLOGS SCN AND 时间戳）

#表空间和数据文件记录（文件名，数据文件检查点，读/写状态，离线范围）

#关于REDO线程信息（当前连机重做日志）

#过去RMAN的备份记录

#关于损坏文件块的信息

The recovery process for datafiles is in part guided by status information in the control

file, such as the database checkpoints, current online redo log file, and the datafile

header checkpoints for the datafiles. Loss of the control file makes recovery from a

data loss much more difficult.

在控制文件中数据文件的还原进程是引导状态信息的一部份。像数据库检查点，当前的

在线重做日志文件，和数据文件头对于数据文件的检查点。控制文件的丢失将使得一个

数据的丢失的还原更加的困难。

In general, when data in a datafile is updated, "before images" of that data are written

into undo segments. If a transaction is rolled back, this undo information can be used

to restore the original datafile contents.

简而言之，当数据在数据文件中被更新时，数据文件的“前镜像”被写入UNDO段。如果一个

事务回滚，这个UNDO信息可以用于恢复原始的数据文件的内容。

In the context of recovery, the undo information is used to undo the effects of

uncommitted transactions, once all the datafile changes from the redo logs have been

applied to the datafiles. The database is actually opened before the undo is applied.

You should not have to concern yourself with undo segments or manage them directly

as part of your backup and recovery process.

在还原的背景下，UNDO信息用于回滚未提交事务的影响。一旦所有的数据文件的改变从

REDO日志已经写入了数据文件。数据库在UNDO应用前是打开状态的。你不能自己关注UNDO

段或者直接管理它们作为备份和还原进程的一部份。