【恢复，1】 redo 日志恢复的各种情况

Recovering After the Loss of Online Redo Log Files

If a media failure has affected the online redo logs of a database, then the appropriate
recovery procedure depends on the following considerations:

The configuration of the online redo log: mirrored or non-mirrored
The type of media failure: temporary or permanent
The types of online redo log files affected by the media failure: current, active, unarchived, or inactive

Table 30-3 displays V$LOG status information that can be crucial
in a recovery situation involving online redo logs.

Table 30-3 STATUS Column of V$LOG

Status	Description
`UNUSED`	The online redo log has never been written to.
`CURRENT`	The online redo log is active, that is, needed for instance recovery, and it is the log to which the database is currently writing. The redo log can be open or closed.
`ACTIVE`	The online redo log is active, that is, needed for instance recovery, but is not the log to which the database is currently writing. It may be in use for block recovery, and may or may not be archived.
`CLEARING`	The log is being re-created as an empty log after an `ALTER` `DATABASE` `CLEAR` `LOGFILE` statement. After the log is cleared, then the status changes to `UNUSED`.
`CLEARING_CURRENT`	The current log is being cleared of a closed thread. The log can stay in this status if there is some failure in the switch such as an I/O error writing the new log header.
`INACTIVE`	The log is no longer needed for instance recovery. It may be in use for media recovery, and may or may not be archived.

Recovering After Losing a Member of a Multiplexed Online Redo Log Group

You can recover after losing a member of a multiplexed online redo log group. The database continues to function as usual during the following conditions:

If the online redo log of a database is multiplexed, and if at least one member of each online redo log group is not affected by the media failure, then the database continues functioning as usual, but error messages are written to
the log writer trace file and the alert_SID.log of the database.

You can resolve the problem of a missing member of a multiplexed online redo log group by taking one of the following actions:

If the hardware problem is temporary, then correct it. The log writer process accesses the previously unavailable online redo log files as if the problem never existed.
If the hardware problem is permanent, then drop the damaged member and add a new member by using the following procedure.

Note:

The newly added member provides no redundancy until the log group is reused.

Locate the file name of the damaged member in V$LOGFILE. The status is INVALID if the file is inaccessible:

SELECT GROUP#, STATUS, MEMBER

FROM V$LOGFILE

WHERE STATUS='INVALID';

GROUP#    STATUS       MEMBER

-------   -----------  ---------------------

0002      INVALID      /disk1/oradata/trgt/redo02.log

Drop the damaged member. For example, to drop member redo02.log from group 2, issue the following statement:
```
ALTER DATABASE DROP LOGFILE MEMBER '/disk1/oradata/trgt/redo02.log';
```
Add a new member to the group. For example, to add redo02.log to group 2, issue the following statement:
```
ALTER DATABASE ADD LOGFILE MEMBER '/disk1/oradata/trgt/redo02b.log'

  TO GROUP 2;
```
If the file that you want to add exists, then it must be the same size as the other group members, and you must specify the REUSE option. For example:
```
ALTER DATABASE ADD LOGFILE MEMBER '/disk1/oradata/trgt/redo02b.log'

  REUSE TO GROUP 2;
```

Recovering After Losing All Members of an Online Redo Log Group

丢失online redo 日志组全部成员后恢复

If a media failure damages all
members of an online redo log group, then different scenarios can occur depending on the type of online redo log group affected by the failure and the archiving mode of the database.

假设介质故障损坏全部的日志组的成员，不同场景下发生的失败影响依赖于日志组类型和数据库的归档模式。

If the damaged online redo log group is current and active, then it is needed for crash recovery; otherwise, it is not. Table
30-4 outlines the various recovery scenarios.

假设损坏的当前日志组时current和active，那么须要做故障恢复。

Table 30-4 Recovering After the Loss of an Online Redo Log Group

If the Group Is...	Then...	And You Should...
Inactive	It is not needed for crash recovery	Clear the archived or unarchived group.
Active	It is needed for crash recovery	Attempt to issue a checkpoint and clear the log; if impossible, then you must either use Flashback Database or restore a backup and perform incomplete recovery up to the most recent available redo log. 试着运行checkpoint和clear日志，假设不能，那么你必需要么使用闪回数据库要么使用转储备份运行不全然恢复到近期可用的redo log。
Current	It is the redo log that the database is currently writing to	Attempt to clear the log; if impossible, then you must either use Flashback Database or restore a backup and perform incomplete recovery up to the most recent available redo log.

If the Group Is...

Then...

And You Should...

Inactive

It is not needed for crash recovery

Clear the archived or unarchived group.

Active

It is needed for crash recovery

Attempt to issue a checkpoint and clear the log; if impossible, then you must either use Flashback Database or restore a backup and perform incomplete recovery up to the most recent available redo
log.

试着运行checkpoint和clear日志，假设不能，那么你必需要么使用闪回数据库要么使用转储备份运行不全然恢复到近期可用的redo log。

Current

It is the redo log that the database is currently writing to

Attempt to clear the log; if impossible, then you must either use Flashback Database or restore a backup and perform incomplete recovery up to the most recent available redo log.

To determine whether the damaged group is active or inactive.

确定损坏的组是否是acitve 或inactive

Locate the file name of the lost redo log in V$LOGFILE and then look for the group number corresponding to it. For
example, enter:

SELECT GROUP#, STATUS, MEMBER FROM V$LOGFILE;

GROUP#    STATUS       MEMBER

-------   -----------  ---------------------

0001                    /oracle/dbs/log1a.f

0001                    /oracle/dbs/log1b.f

0002      INVALID       /oracle/dbs/log2a.f

0002      INVALID       /oracle/dbs/log2b.f

0003                    /oracle/dbs/log3a.f

0003                    /oracle/dbs/log3b.f

Determine which groups are active.

For example, execute the following SQL query (sample output included):

SELECT GROUP#, MEMBERS, STATUS, ARCHIVED

FROM V$LOG;

GROUP#  MEMBERS           STATUS     ARCHIVED

------  -------           ---------  -----------

 0001   2                 INACTIVE   YES

 0002   2                 ACTIVE     NO

 0003   2                 CURRENT    NO

Perform one of the following actions:
- If the affected group is inactive, then follow the procedure in "Losing an
  Inactive Online Redo Log Group".
- If the affected group is active (as in the preceding example), then follow the procedure in "Losing
  an Active Online Redo Log Group".

Losing an Inactive Online Redo Log Group

丢失inactive 日志组

If all members of an online
redo log group with INACTIVE status are damaged, then the procedure depends on whether you can fix the media problem that damaged the inactive redo log group. If the failure is temporary, then fix the problem. The log writer
can reuse the redo log group when required. If the failure is permanent, then the damaged inactive online redo log group eventually halts normal database operation. Reinitialize the damaged group manually by issuing the ALTER DATABASE CLEAR LOGFILE statement as described in this section.

inactive状态的日志组全部成员损坏。那么程序依赖于你是否能定位inactive日志组的介质问题(日志文件是否损坏)，假设介质故障是暂时的，log writer 能又一次使用日志组。假设介质故障是永久的，那么损坏的inactive日志组终于会停止正常的数据库操作。通过运行alter database CLEAR LOGFILE语句手动的又一次初始化损坏的日志组。

Clearing Inactive, Archived Redo

清除inactive 已归档的日志

You can clear an inactive redo log group when the database is open or closed. The procedure depends on whether the damaged group has been archived.

数据库开启或关闭都能 clear inactive 日志组。依赖于损坏的日志组是否归档。

To clear an inactive, online redo log group that has been archived:

If the database is shut down, then start a new instance and mount the database:
```
STARTUP MOUNT
```
Reinitialize the damaged log group. For example, to clear redo log group 2, issue the following statement:
```
ALTER DATABASE CLEAR LOGFILE GROUP 2;
```

Clearing Inactive, Unarchived Redo

清除 inactive 未归档的日志

Clearing a not-yet-archived redo log allows it to be reused without archiving it. This action makes backups unusable if they were started before the last change in the log, unless the file was taken offline before the first change
in the log. Hence, if you need the cleared log file for recovery of a backup, then you cannot recover that backup. Clearing a not-yet-archived-redo-log, prevents complete recovery from backups due to the missing log.

清除的没有归档的日志同意被被重用。

假设日志的最后一次改变之前開始，这个操作会导致备份不可用，除非日志文件在第一次改变之前被置为offline。

此后。假设你须要被清除的日志去做备份恢复，是不能备份恢复的。

To clear an inactive, online redo log group that has not been archived:

清除一个inactive ，日志没有归档：

If the database is shut down, then start a new instance and mount the database:
```
SQL> STARTUP MOUNT
```
Clear the log using the UNARCHIVED keyword.

For example, to clear log group 2, issue the following SQL statement:
```
SQL> ALTER DATABASE CLEAR UNARCHIVED LOGFILE GROUP 2;
```
If there is an offline data file that requires the cleared log to bring it online, then the keywords UNRECOVERABLE DATAFILE are required. The data file must be
dropped because the redo logs necessary to bring the data file online are being cleared, and there is no copy of it. For example, enter:

假设有一个数据文件offline，须要清除日志后使数据文件上线，那么此时须要使用keywordUNRECOVERABLE DATAFILE.数据文件必须被删除由于
```
SQL> ALTER DATABASE CLEAR UNARCHIVED LOGFILE GROUP 2 UNRECOVERABLE DATAFILE;
```
Immediately back up all data files in the database with an operating system utility, so that you have a backup you can use for complete recovery without relying on the cleared log group. For example, enter:

马上备份全部的数据文件使用操作系统工具，以至于你有一个不用依赖被清除的日志组就能做全然恢复的备份。
```
% cp /disk1/oracle/dbs/*.dbf /disk2/backup
```
Back up the database's control file with the ALTER DATABASE statement. For example, enter:
```
SQL> ALTER DATABASE BACKUP CONTROLFILE TO '/oracle/dbs/cf_backup.f';
```

Failure of CLEAR LOGFILE Operation

失败的清除日志文件操作

The ALTER DATABASE CLEAR LOGFILE statement
can fail with an I/O error due to media failure when it is not possible to:

alter database clear logfile 语句假设不能做以下的操作会失败因为介质故障导致的io错误，

Relocate the redo log file onto alternative media by re-creating it under the currently configured redo log file name

通过当前配置的日志文件的名称又一次创建日志文件迁移日志日志到另外的介质上
Reuse the currently configured log file name to re-create the redo log file because the name itself is invalid or unusable (for example, due to media failure)

又一次使用当前的配置日志文件的名称去又一次创建日志文件由于他自己的名字是无效的或者不可用。

In these cases, the ALTER DATABASE CLEAR LOGFILE statement
(before receiving the I/O error) would have successfully informed the control file that the log was being cleared and did not require archiving. The I/O error occurred at the step in which the CLEAR LOGFILE statement
attempted to create the new redo log file and write zeros to it. This fact is reflected in V$LOG.CLEARING_CURRENT.

在这些情况下，alter database clear logfile 语句

Losing an Active Online Redo Log Group

If the database is still running and the lost active redo log is not the current log, then issue the ALTER SYSTEM CHECKPOINT statement.
If the operation is successful, then the active redo log becomes inactive, and you can follow the procedure in "Losing
an Inactive Online Redo Log Group". If the operation is unsuccessful, or if your database has halted, then perform one of procedures in this section, depending on the archiving mode.

The current log is the one LGWR is currently writing to. If a LGWR I/O operation fails, then LGWR terminates and the instance fails. In this case, you must restore
a backup, perform incomplete recovery, and open the database with the RESETLOGS option.

Recovering from the Loss of Active Logs in NOARCHIVELOG Mode

In this scenario, the database archiving mode is NOARCHIVELOG.

To recover from the loss of an active online log group in NOARCHIVELOG mode:

If the media failure is temporary, then correct the problem so that the database can reuse the group when required.
Restore the database from a consistent, whole database backup (data files and control files). For example, enter:
```
% cp /disk2/backup/*.dbf $ORACLE_HOME/oradata/trgt/
```
Mount the database:
```
STARTUP MOUNT
```
Because online redo logs are not backed up, you cannot restore them with the data files and control files. To allow the database to reset the online redo logs, you must first mimic incomplete recovery:
```
RECOVER DATABASE UNTIL CANCEL

CANCEL
```
Open the database using the RESETLOGS option:
```
ALTER DATABASE OPEN RESETLOGS;
```
Shut down the database consistently. For example, enter:
```
SHUTDOWN IMMEDIATE
```
Make a whole database backup.

If the media failure is temporary, then correct the problem so that the database can reuse the group when required. If
the media failure is not temporary, then use the following procedure.

Recovering from Loss of Active Logs in ARCHIVELOG Mode

In this scenario, the database archiving mode is ARCHIVELOG.

To recover from loss of an active online redo log group in ARCHIVELOG mode:

Begin incomplete media recovery, recovering up through the log before the damaged log.
Ensure that the current name of the lost redo log can be used for a newly created file. If not, then rename the members of the damaged online redo log group to a new location. For example, enter:
```
ALTER DATABASE RENAME FILE "/disk1/oradata/trgt/redo01.log" TO "/tmp/redo01.log";

ALTER DATABASE RENAME FILE "/disk1/oradata/trgt/redo02.log" TO "/tmp/redo02.log";
```
Open the database using the RESETLOGS option:
```
ALTER DATABASE OPEN RESETLOGS;
```
Note:

All updates executed from the end point of the incomplete recovery to the present must be re-executed.

Loss of Multiple Redo Log Groups

If you have lost multiple groups of the online redo log, then use the recovery method for the most difficult log to recover. The order of difficulty, from most difficult to least
difficult, is as follows:

The current online redo log
An active online redo log
An unarchived online redo log
An inactive online redo log

来源： <http://docs.oracle.com/cd/E11882_01/backup.112/e10642/osadvsce.htm#BRADV99985>