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Managing Archive redo log

原创 Oracle 作者:caisanpx 时间:2014-03-26 14:33:07 0 删除 编辑
1. Archive redo log

Oracle Database lets you save filled groups of redo log files to one or more offline destinations, known collectively as the archived redo log. The process of turning redo log files into archived redo log files is called archiving. This process is only possible if the database is running in ARCHIVELOG mode. You can choose automatic or manual archiving.

You can use archived redo logs to
1.Recover a database

2.Update a standby database
3.Get information about the history of a database using the LogMiner utility


2.Running a Database in ARCHIVELOG Mode

When you run a database in ARCHIVELOG mode, you enable the archiving of the redo log. The database control file indicates that a group of filled redo log files cannot be reused by LGWR until the group is archived. A filled group becomes available for archiving immediately after a redo log switch occurs.
The archiving of filled groups has these advantages:

1.A database backup, together with online and archived redo log files, guarantees that you can recover all committed transactions in the event of an operating system or disk failure.
2.If you keep an archived log, you can use a backup taken while the database is open and in normal system use.
3.You can keep a standby database current with its original database by continuously applying the original archived redo logs to the standby.

You can configure an instance to archive filled redo log files automatically, or you can archive manually. For convenience and efficiency, automatic archiving is usually best.  illustrates how the archiver process (ARC0 in this illustration) writes filled redo log files to the database archived redo log.

3.Controlling Archiving

3.1Changing the Database Archiving Mode

To change the archiving mode of the database, use the ALTER DATABASE statement with the ARCHIVELOG or NOARCHIVELOG clause. To change the archiving mode, you must be connected to the database with administrator privileges (AS SYSDBA).

The following steps switch the database archiving mode from NOARCHIVELOG to ARCHIVELOG:

  1. 1.Shut down the database instance.

    SHUTDOWN
    

    An open database must first be closed and any associated instances shut down before you can switch the database archiving mode. You cannot change the mode from ARCHIVELOG to NOARCHIVELOG if any datafiles need media recovery.

  2. 2.Back up the database.

    Before making any major change to a database, always back up the database to protect against any problems. This will be your final backup of the database in NOARCHIVELOG mode and can be used if something goes wrong during the change to ARCHIVELOG mode. See Oracle Database Backup and Recovery User's Guide for information about taking database backups.

  3. 3.Edit the initialization parameter file to include the initialization parameters that specify the destinations for the archived redo log files (see "Setting Initialization Parameters for Archive Destinations").

  4. 4.Start a new instance and mount, but do not open, the database.

    STARTUP MOUNT
    

    To enable or disable archiving, the database must be mounted but not open.

  5. 5.Change the database archiving mode. Then open the database for normal operations.

    ALTER DATABASE ARCHIVELOG;
    ALTER DATABASE OPEN;
    
  6. 6.Shut down the database.

    SHUTDOWN IMMEDIATE
    
  7. 7.Back up the database.

    Changing the database archiving mode updates the control file. After changing the database archiving mode, you must back up all of your database files and control file. Any previous backup is no longer usable because it was taken in NOARCHIVELOG mode.

    See Also:

    Oracle Real Application Clusters Administration and Deployment Guide for more information about switching the archiving mode when using Real Application Clusters

3.2Performing Manual Archiving

As mentioned in "Running a Database in ARCHIVELOG Mode", for convenience and efficiency, automatic archiving is usually best. However, you can configure your database for manual archiving only. To operate your database in manual archiving mode, follow the procedure described in "Changing the Database Archiving Mode", but replace the ALTER DATABASE statement in step 5 with the following statement:

ALTER DATABASE ARCHIVELOG MANUAL;

When you operate your database in manual ARCHIVELOG mode, you must archive inactive groups of filled redo log files or your database operation can be temporarily suspended. To archive a filled redo log group manually, connect with administrator privileges. Ensure that the database is either mounted or open. Use the ALTER SYSTEM statement with the ARCHIVE LOG clause to manually archive filled redo log files. The following statement archives all unarchived log files:

ALTER SYSTEM ARCHIVE LOG ALL;

When you use manual archiving mode, you cannot specify any standby databases in the archiving destinations.

Even when automatic archiving is enabled, you can use manual archiving for such actions as rearchiving an inactive group of filled redo log members to another location. In this case, it is possible for the instance to reuse the redo log group before you have finished manually archiving, and thereby overwrite the files. If this happens, the database writes an error message to the alert log.

3.3Adjusting the Number of Archiver Processes

The following statement configures the database to start six ARCn processes upon startup:

ALTER SYSTEM SET LOG_ARCHIVE_MAX_PROCESSES=6;

The statement also has an immediate effect on the currently running instance. It increases or decreases the current number of running ARCn processes to six.


4.Specifying Archive Destinations

4.1Setting Initialization Parameters for Archive Destinations

The following table summarizes the archive destination alternatives, which are further described in the sections that follow.

Method Initialization Parameter Host Example
1 LOG_ARCHIVE_DEST_n

where:

n is an integer from 1 to 31. Archive destinations 1 to 10 are available for local or remote locations. Archive destinations 11 to 31 are available for remote locations only.

Local or remote LOG_ARCHIVE_DEST_1 = 'LOCATION=/disk1/arc'

LOG_ARCHIVE_DEST_2 = 'LOCATION=/disk2/arc'

LOG_ARCHIVE_DEST_3 = 'SERVICE=standby1'

2 LOG_ARCHIVE_DEST and

LOG_ARCHIVE_DUPLEX_DEST

Local only LOG_ARCHIVE_DEST = '/disk1/arc'

LOG_ARCHIVE_DUPLEX_DEST = '/disk2/arc'



Method 1: Using the LOG_ARCHIVE_DEST_n Parameter

Use the LOG_ARCHIVE_DEST_n parameter (where n is an integer from 1 to 31) to specify from one to 31 different destinations for archived logs. Each numerically suffixed parameter uniquely identifies an individual destination.

You specify the location for LOG_ARCHIVE_DEST_n using the keywords explained in the following table:

Keyword Indicates Example
LOCATION A local file system location or Oracle ASM disk group LOG_ARCHIVE_DEST_n = 'LOCATION=/disk1/arc'

LOG_ARCHIVE_DEST_n = 'LOCATION=+DGROUP1'

LOCATION The Fast Recovery Area LOG_ARCHIVE_DEST_n = 'LOCATION=USE_DB_RECOVERY_FILE_DEST'
SERVICE Remote archival through Oracle Net service name. LOG_ARCHIVE_DEST_n = 'SERVICE=standby1'

If you use the LOCATION keyword, specify one of the following:

  • A valid path name in your operating system's local file system

  • An Oracle ASM disk group

  • The keyword USE_DB_RECOVERY_FILE_DEST to indicate the Fast Recovery Area

If you specify SERVICE, supply a net service name that Oracle Net can resolve to a connect descriptor for a standby database. The connect descriptor contains the information necessary for connecting to the remote database.


Method 2: Using LOG_ARCHIVE_DEST and LOG_ARCHIVE_DUPLEX_DEST

To specify a maximum of two locations, use the LOG_ARCHIVE_DEST parameter to specify a primary archive destination and the LOG_ARCHIVE_DUPLEX_DEST to specify an optional secondary archive destination. All locations must be local. Whenever the database archives a redo log, it archives it to every destination specified by either set of parameters.

Perform the following steps the use method 2:

  1. Specify destinations for the LOG_ARCHIVE_DEST and LOG_ARCHIVE_DUPLEX_DEST parameter (you can also specify LOG_ARCHIVE_DUPLEX_DEST dynamically using the ALTER SYSTEM statement). For example, enter:

    LOG_ARCHIVE_DEST = '/disk1/archive'
    LOG_ARCHIVE_DUPLEX_DEST = '/disk2/archive'
    
  2. Set the LOG_ARCHIVE_FORMAT initialization parameter as described in step 2 for method 1.

Note:

If you configure a Fast Recovery Area (by setting the DB_RECOVERY_FILE_DEST and DB_RECOVERY_FILE_DEST_SIZE parameters) and do not specify any local archive destinations, the database automatically selects the Fast Recovery Area as a local archive destination and sets LOG_ARCHIVE_DEST_1 to USE_DB_RECOVERY_FILE_DEST.

WARNING:

You must ensure that there is sufficient disk space at all times for archive log destinations. If the database encounters a disk full error as it attempts to archive a log file, a fatal error occurs and the database stops responding. You can check the alert log for a disk full message.


5.Controlling Trace Output Generated by the Archivelog Process

Background processes always write to a trace file when appropriate. (See the discussion of this topic in "Monitoring Errors with Trace Files and the Alert Log".) In the case of the archivelog process, you can control the output that is generated to the trace file. You do this by setting the LOG_ARCHIVE_TRACE initialization parameter to specify a trace level. The following values can be specified:

Trace Level Meaning
0 Disable archivelog tracing. This is the default.
1 Track archival of redo log file.
2 Track archival status for each archivelog destination.
4 Track archival operational phase.
8 Track archivelog destination activity.
16 Track detailed archivelog destination activity.
32 Track archivelog destination parameter modifications.
64 Track ARCn process state activity.
128 Track FAL (fetch archived log) server related activities.
256 Supported in a future release.
512 Tracks asynchronous LGWR activity.
1024 RFS physical client tracking.
2048 ARCn/RFS heartbeat tracking.
4096 Track real-time apply
8192 Track redo apply activity (media recovery or physical standby)

You can combine tracing levels by specifying a value equal to the sum of the individual levels that you would like to trace. For example, setting LOG_ARCHIVE_TRACE=12, will generate trace level 8 and 4 output. You can set different values for the primary and any standby database.

The default value for the LOG_ARCHIVE_TRACE parameter is 0. At this level, the archivelog process generates appropriate alert and trace entries for error conditions.

You can change the value of this parameter dynamically using the ALTER SYSTEM statement. The database must be mounted but not open. For example:

ALTER SYSTEM SET LOG_ARCHIVE_TRACE=12;

Changes initiated in this manner will take effect at the start of the next archiving operation.


6.Archived Redo Logs Views

Several dynamic performance views contain useful information about archived redo logs, as summarized in the following table.

Dynamic Performance View Description
V$DATABASE Shows if the database is in ARCHIVELOG or NOARCHIVELOG mode and if MANUAL (archiving mode) has been specified.
V$ARCHIVED_LOG Displays historical archived log information from the control file. If you use a recovery catalog, the RC_ARCHIVED_LOG view contains similar information.
V$ARCHIVE_DEST Describes the current instance, all archive destinations, and the current value, mode, and status of these destinations.
V$ARCHIVE_PROCESSES Displays information about the state of the various archive processes for an instance.
V$BACKUP_REDOLOG Contains information about any backups of archived logs. If you use a recovery catalog, the RC_BACKUP_REDOLOG contains similar information.
V$LOG Displays all redo log groups for the database and indicates which need to be archived.
V$LOG_HISTORY Contains log history information such as which logs have been archived and the SCN range for each archived log.



More details:http://docs.oracle.com/cd/E11882_01/server.112/e25494/archredo.htm#ADMIN008


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