This page is a copy from http://www.orafaq.com/wiki/E-Business_Suite

Please note: This page is not maintained anymore. Please visit the new and improved page at http://www.orafaq.com/wiki/E-Business_Suite

Oracle Backup and Recovery FAQ

$Date: 29-Jul-2005 $
$Revision: 2.04 $
$Author: Frank Naudé $

 

The wise one once said: Backup early and often... database crashes can happen at any time for any reason!

Topics

  • Why and when should I backup my database?
  • What strategies are available for backing-up an Oracle database?
  • What is the difference between on-line and off-line backups?
  • What is the difference between restoring and recovering?
  • How does one backup a database using the export utility?
  • How does one do off-line database backups?
  • How does one do on-line database backups?
  • How does one backup a database using RMAN?
  • How does one put a database into ARCHIVELOG mode?
  • How does one backup archived log files?
  • Does Oracle write to data files in hot/begin backup mode?
  • My database was terminated while in BACKUP MODE, do I need to recover?
  • My database is down and I cannot restore. What now?
  • I've lost my REDOLOG files, how can I get my DB back?
  • I've lost some Rollback Segments, how can I get my DB back?
  • What are the differences between EBU and RMAN?
  • How does one create an RMAN recovery catalog?
  • What are the common RMAN errors (with solutions)?
  • What third party tools can be used with Oracle EBU/ RMAN?
  • Where can one get more info about Oracle Backup and Recovery?

  • Back to top of file

    What strategies are available for backing-up an Oracle database?

    The following methods are valid for backing-up an Oracle database:

    It is advisable to use more than one of these methods to backup your database. For example, if you choose to do on-line database backups, also cover yourself by doing database exports. Also test ALL backup and recovery scenarios carefully. It is better to be safe than sorry.

    Regardless of your strategy, also remember to backup all required software libraries, parameter files, password files, etc. If your database is in ARCHIVELOG mode, you also need to backup archived log files.

     

  • Back to top of file

    What is the difference between on-line and off-line backups?

    A hot backup is a backup performed while the database is on-line and available for read/write. Except for Oracle exports, one can only do on-line backups when running in ARCHIVELOG mode.

    A cold backup is a backup performed while the database is off-line and unavailable to its users.

     

  • Back to top of file

    What is the difference between restoring and recovering?

    Restoring involves copying backup files from secondary storage (backup media) to disk. This can be done to replace damaged files or to copy/move a database to a new location.

    Recovery is the process of applying redo logs to the database to roll it forward. One can roll-forward until a specific point-in-time (before the disaster occurred), or roll-forward until the last transaction recorded in the log files.

    	 sql> connect SYS as SYSDBA
    	 sql> RECOVER DATABASE UNTIL TIME '2001-03-06:16:00:00' USING BACKUP CONTROLFILE;
    

     

  • Back to top of file

    How does one backup a database using the export utility?

    Oracle exports are "logical" database backups (not physical) as they extract data and logical definitions from the database into a file. Other backup strategies normally back-up the physical data files.

    One of the advantages of exports is that one can selectively re-import tables, however one cannot roll-forward from a restored export file. To completely restore a database from an export file one practically needs to recreate the entire database.

    Always do full system level exports (FULL=YES). Full exports include more information about the database in the export file than user level exports. For more information about the Oracle export and import utilities, see the Import/ Export FAQ.

     

  • Back to top of file

    How does one do off-line database backups?

    Shut down the database from sqlplus or server manager. Backup all files to secondary storage (eg. tapes). Ensure that you backup all data files, all control files and all log files. When completed, restart your database.

    Do the following queries to get a list of all files that needs to be backed up:

    	 select name from sys.v_$datafile;
    	 select member from sys.v_$logfile;
    	 select name from sys.v_$controlfile;
    
    Sometimes Oracle takes forever to shutdown with the "immediate" option. As workaround to this problem, shutdown using these commands:
    	 alter system checkpoint;
    	 shutdown abort
    	 startup restrict
    	 shutdown immediate
    
    Note that if you database is in ARCHIVELOG mode, one can still use archived log files to roll forward from an off-line backup. If you cannot take your database down for a cold (off-line) backup at a convenient time, switch your database into ARCHIVELOG mode and perform hot (on-line) backups.

     

  • Back to top of file

    How does one do on-line database backups?

    Each tablespace that needs to be backed-up must be switched into backup mode before copying the files out to secondary storage (tapes). Look at this simple example.
    	 ALTER TABLESPACE xyz BEGIN BACKUP;
    	 ! cp xyfFile1 /backupDir/
    	 ALTER TABLESPACE xyz END BACKUP;
    
    It is better to backup tablespace for tablespace than to put all tablespaces in backup mode. Backing them up separately incurs less overhead. When done, remember to backup your control files. Look at this example:
    	 
    	ALTER SYSTEM SWITCH LOGFILE;   -- Force log switch to update control file headers 	 
    	ALTER DATABASE BACKUP CONTROLFILE TO '/backupDir/control.dbf';
    
    NOTE: Do not run on-line backups during peak processing periods. Oracle will write complete database blocks instead of the normal deltas to redo log files while in backup mode. This will lead to excessive database archiving and even database freezes.

     

  • Back to top of file

    How does one backup a database using RMAN?

    The biggest advantage of RMAN is that it will only backup used space in the database. Rman doesn't put tablespaces in backup mode, saving on redo generation overhead. RMAN will re-read database blocks until it gets a consistent image of it. Look at this simple backup example.
    	 rman target sys/*** nocatalog 
    	 run { 
    	    allocate channel t1 type disk;
    	    backup 
    	       format '/app/oracle/db_backup/%d_t%t_s%s_p%p'
    	       ( database ); 
    	    release channel t1; 
    	 }
    
    Example RMAN restore:
    	 rman target sys/*** nocatalog 
    	 run {
    	    allocate channel t1 type disk;
    	    # set until time 'Aug 07 2000 :51';
    	    restore tablespace users; 
    	    recover tablespace users; 
    	    release channel t1; 
    	 }
    
    The examples above are extremely simplistic and only useful for illustrating basic concepts. By default Oracle uses the database controlfiles to store information about backups. Normally one would rather setup an RMAN catalog database to store RMAN metadata in. Read the Oracle Backup and Recovery Guide before implementing any RMAN backups.

    Note: RMAN cannot write image copies directly to tape. One needs to use a third-party media manager that integrates with RMAN to backup directly to tape. Alternatively one can backup to disk and then manually copy the backups to tape.

     

  • Back to top of file

    How does one put a database into ARCHIVELOG mode?

    The main reason for running in archivelog mode is that one can provide 24-hour availability and guarantee complete data recoverability. It is also necessary to enable ARCHIVELOG mode before one can start to use on-line database backups.

    To enable ARCHIVELOG mode, simply change your database startup command script, and bounce the database:

            SQLPLUS> connect sys as sysdba
            SQLPLUS> startup mount exclusive;
            SQLPLUS> alter database archivelog;
            SQLPLUS> archive log start;
            SQLPLUS> alter database open;
    
    NOTE1: Remember to take a baseline database backup right after enabling archivelog mode. Without it one would not be able to recover. Also, implement an archivelog backup to prevent the archive log directory from filling-up.

    NOTE2: ARCHIVELOG mode was introduced with Oracle V6, and is essential for database point-in-time recovery. Archiving can be used in combination with on-line and off-line database backups.

    NOTE3: You may want to set the following INIT.ORA parameters when enabling ARCHIVELOG mode: log_archive_start=TRUE, log_archive_dest=..., and log_archive_format=...

    NOTE4: You can change the archive log destination of a database on-line with the ARCHIVE LOG START TO 'directory'; statement. This statement is often used to switch archiving between a set of directories.

    NOTE5: When running Oracle Real Application Server (RAC), you need to shut down all nodes before changing the database to ARCHIVELOG mode. See the RAC FAQ for more details.

     

  • Back to top of file

    How does one backup archived log files?

    One can backup archived log files using RMAN or any operating system backup utility. Remember to delete files after backing them up to prevent the archive log directory from filling up. If the archive log directory becomes full, your database will hang! Look at this simple RMAN backup script:
    	RMAN> run {
    	2> allocate channel dev1 type disk;
    	3> backup
    	4>   format '/app/oracle/arch_backup/log_t%t_s%s_p%p'
    	5>   (archivelog all delete input);
    	6> release channel dev1;
    	7> }
    
  • Back to top of file

    Does Oracle write to data files in begin/hot backup mode?

    Oracle will stop updating file headers, but will continue to write data to the database files even if a tablespace is in backup mode.

    In backup mode, Oracle will write out complete changed blocks to the redo log files. Normally only deltas (changes) are logged to the redo logs. This is done to enable reconstruction of a block if only half of it was backed up (split blocks). Because of this, one should notice increased log activity and archiving during on-line backups.

     

  • Back to top of file

    My database was terminated while in BACKUP MODE, do I need to recover?

    If a database was terminated while one of its tablespaces was in BACKUP MODE (ALTER TABLESPACE xyz BEGIN BACKUP;), it will tell you that media recovery is required when you try to restart the database. The DBA is then required to recover the database and apply all archived logs to the database. However, from Oracle7.2, you can simply take the individual datafiles out of backup mode and restart the database.
    	 ALTER DATABASE DATAFILE '/path/filename' END BACKUP;
    
    One can select from V$BACKUP to see which datafiles are in backup mode. This normally saves a significant amount of database down time. See script end_backup2.sql in the script section of this FAQ.

    Thiru Vadivelu contributed the following:

    From Oracle9i onwards, the following command can be used to take all of the datafiles out of hotbackup mode:

    	ALTER DATABASE END BACKUP;
    
    The above command needs to be issued when the database is mounted.

     

  • Back to top of file

    My database is down and I cannot restore. What now?

    Recovery without any backup is normally not supported, however, Oracle Consulting can sometimes extract data from an off-line database using a utility called DUL (Disk UnLoad). This utility reads data in the data files and unloads it into SQL*Loader or export dump files. DUL does not care about rollback segments, corrupted blocks, etc, and can thus not guarantee that the data is not logically corrupt. It is intended as an absolute last resort and will most likely cost your company a lot of money!!!

     

    "Life is DUL without it!"

     

  • Back to top of file

    I've lost my REDOLOG files, how can I get my DB back?

    The following INIT.ORA parameter may be required if your current redologs are corrupted or blown away. Caution is advised when enabling this parameter as you might end-up losing your entire database. Please contact Oracle Support before using it.
            _allow_resetlogs_corruption = true
    

     

  • Back to top of file

    I've lost some Rollback Segments, how can I get my DB back?

    Re-start your database with the following INIT.ORA parameter if one of your rollback segments is corrupted. You can then drop the corrupted rollback segments and create it from scratch. Caution is advised when enabling this parameter as uncommitted transactions will be marked as committed. One can very well end up with lost or inconsistent data!!! Please contact Oracle Support before using it.
            _corrupted_rollback_segments = (rbs01,rbs01,rbs03,rbs04)
    

     

  • Back to top of file

    What are the differences between EBU and RMAN?

    Enterprise Backup Utility (EBU) is a functionally rich, high performance interface for backing up Oracle7 databases. It is sometimes referred to as OEBU for Oracle Enterprise Backup Utility.

    The Oracle Recovery Manager (RMAN) utility that ships with Oracle8 and above is similar to Oracle7's EBU utility. However, there is no direct upgrade path from EBU to RMAN.

     

  • Back to top of file

    How does one create an RMAN recovery catalog?

    Start by creating a database schema (usually called rman). Assign an appropriate tablespace to it and grant it the recovery_catalog_owner role. Look at this example:
    	sqlplus sys
    	SQL> create user rman identified by rman;
    	SQL> alter user rman default tablespace tools temporary tablespace temp;
    	SQL> alter user rman quota unlimited on tools;
    	SQL> grant connect, resource, recovery_catalog_owner to rman;
    	SQL> exit;
    
    Next, log in to rman and create the catalog schema. Prior to Oracle 8i this was done by running the catrman.sql script.
    	rman catalog rman/rman
    	RMAN> create catalog tablespace tools;
    	RMAN> exit;
    
    You can now continue by registering your databases in the catalog. Look at this example:
     
    	rman catalog rman/rman target backdba/backdba
    	RMAN> register database;
    

     

  • Back to top of file

    What are the common RMAN errors (with solutions)?

    Some of the common RMAN errors are:

     

  • RMAN-20242: Specification does not match any archivelog in the recovery catalog.

     

     

  • RMAN-06089: archived log xyz not found or out of sync with catalog

     

     

  • Back to top of file

    What third party tools can be used with Oracle EBU/ RMAN?

    The following Media Management Software Vendors have integrated their media management software packages with Oracle Recovery Manager and Oracle7 Enterprise Backup Utility. The Media Management Vendors will provide first line technical support for the integrated backup/recover solutions.

     

  • Back to top of file

    Where can one get more info about Oracle Backup and Recovery?

     

  • Back to top of file

     
    HOME | ASK QUESTION | ADD FAQ | SEARCH | E-MAIL US