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DBA2 - Enable Archived Redo Log

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• DBA2 - Enable Archived Redo Log
  • Creating Archived Redo Log Files
    • Archiver (ARCn) Process
    • Archived Redo Log Files: Naming and Destinations
  • Configuring ARCHIVELOG Mode
  • Lab: ArchivedLog mode
    • Query whether in ARCHIVELOG mode
    • Enable ARCHIVELOG mode
    • Stop and Disable ArchiveLog mode

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Creating Archived Redo Log Files

• Place your database in ARCHIVELOG mode.

  • In all cases, you will be able to recover the database either completely or incompletely depending on which database files have been damaged or lost.

• Configure redundant archive log destinations.

  • In cases where you lost archive log files and you need them to recover the database, you will be able to perform an incomplete recovery, unless you have a duplicate version of the archive log in another destination.

• The Oracle Database server treats the online redo log groups as a circular buffer in which to store transaction information, filling one group and then moving on to the next.

  • After all groups have been written to, the Oracle Database server begins overwriting information in the first log group.

• archived redo log files

  • a copy of the online redo log group before allowing it to be overwritten.

• To facilitate the creation of archived redo log files:

  1. Specify a naming convention for your archived redo log files.
  2. Specify a destination or destinations for storing your archived redo log files.
  3. Place the database in ARCHIVELOG mode.
  • Note: Steps 1 and 2 are not necessary if you are using a fast recovery area.

• The destination should exist before placing the database in ARCHIVELOG mode. When a directory is specified as a destination, there should be a slash at the end of the directory name.

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Archiver (ARCn) Process

• log switch:

  • When an online redo log group gets filled, the Oracle Database server begins writing to the next online redo log group.
  • the process of switching from one online redo log group to another

• Archiver (ARCn):

  • Automatically archives online redo log files when the database is in ARCHIVELOG mode
  • Preserves a record of all changes made to the database

• The ARCn process initiates archiving of the filled log group at every log switch.

  • It automatically archives the online redo log group before the log group can be reused so that all the changes made to the database are preserved.
  • This enables recovery of the database to the point of failure even if a disk drive is damaged.

• One of the important decisions that a DBA must make is whether to configure the database to operate in ARCHIVELOG mode or in NOARCHIVELOG mode.

  • In NOARCHIVELOG mode, the online redo log files are overwritten each time a log switch occurs.
  • In ARCHIVELOG mode, inactive groups of filled online redo log files must be archived before they can be used again.

• Notes:

  • ARCHIVELOG mode is essential for most backup strategies.
  • If the archived redo log file destination fills up or cannot be written to, the database will eventually come to a halt停机.
    • Remove archived redo log files from the archived redo log file destination and the database will resume operations.移除存档

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Archived Redo Log Files: Naming and Destinations

• Destination:

  • single destination:
    • parameterLOG_ARCHIVE_DEST
  • Multiple destination:
    • parameters LOG_ARCHIVE_DEST_n
    • can be 31 different destinations.
    • may be local (a directory) or remote (an Oracle Net alias for a standby database).
  • If using file system storage, it is recommended that you add multiple locations across different disks. 分散分布
  • If the fast recovery area is enabled, by default DB_RECOVERY_FILE_DEST is specified as an archived redo log file destination.

• Naming:

  • Each archived redo log file must have a unique name to avoid overwriting older log files.

  • To help create unique file names, Oracle Database allows several wildcard characters in the name format:

    • %s: Includes the log sequence number as part of the file name
    • %t: Includes the thread number as part of the file name
    • %r: Includes the resetlogs ID to ensure that the archive log file name remains unique (even after certain advanced recovery techniques that reset log sequence numbers)
    • %d: Includes the database ID as part of the file name
  • best practice:
    • should include %s, %t, and %r as .
    • %d can also be included if multiple databases share the same archive log destination.

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Configuring ARCHIVELOG Mode

• Using SQL commands:

  • 1. Shut down the database instance if it is open.
  • 1. Mount the database.
  • 1. Issue the ALTER DATABASE ARCHIVELOG command.
  • 1. Open the database.

shutdown immediate
startup mount
alter database archivelog;
alter database open;

• can use Enterprise Manager Cloud Control to place the database in ARCHIVELOG mode.

• ARCHIVELOG mode:

  • prevents redo logs from being overwritten until they have been archived.
  • To put the database in ARCHIVELOG mode, the database must be in MOUNT mode.
    • If the database is currently open, you must shut it down cleanly (not abort) and then mount it.
  • recovery is possible until the time of the last commit.
  • Most production databases are operated in ARCHIVELOG mode.
  • Back up your database after switching to ARCHIVELOG mode because your database is recoverable only from the first backup taken in that mode. 模式转变后,必须备份一次; 从转变后第一次备份开始可恢复性.

• NOARCHIVELOG mode

  • Default mode
  • recovery is possible only until the time of the last backup.
    • All transactions made after that backup are lost.

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Lab: ArchivedLog mode

Query whether in ARCHIVELOG mode

archive log list

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Enable ARCHIVELOG mode

• must put the database in MOUNT mode to enable archiving.

shutdown immediate
startup mount
alter database archivelog;
archive log list
alter database open;

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Stop and Disable ArchiveLog mode

-- when the db is up and running
alter system archive log stop;
shutdown immediate

-- Startup in exclusive mount mode.
startup mount restrict
-- if flashback database is enabled, it cannot disable
alter database noarchivelog;
alter database open;
archive log list

--Shutdown Oracle and startup normally
shutdown immediate
startup

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