Backup & Recovery FAQ's

Backup & Recovery FAQ's

Why and when should I backup my database?

Backup and recovery is one of the most important aspects of a DBA's job. If you lose your company's data, you could very well lose your job. Hardware and software can always be replaced, but your data may be irreplaceable!

Normally one would schedule a hierarchy of daily, weekly and monthly backups, however consult with your users before deciding on a backup schedule. Backup frequency normally depends on the following factors:

* Rate of data change/ transaction rate
* Database availability/ Can you shutdown for cold backups?
* Criticality of the data/ Value of the data to the company
* Read-only tablespace needs backing up just once right after you make it read-only
* If you are running in archivelog mode you can backup parts of a database over an extended cycle of days
* If archive logging is enabled one needs to backup archived log files timeously to prevent database freezes
* Etc.
Carefully plan backup retention periods. Ensure enough backup media (tapes) are available and that old backups are expired in-time to make media available for new backups. Off-site vaulting is also highly recommended.
Frequently test your ability to recover and document all possible scenarios. Remember, it's the little things that will get you. Most failed recoveries are a result of organizational errors and miscommunication.

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

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

Export/Import - Exports are "logical" database backups in that they extract logical definitions and data from the database to a file. See the Import/ Export FAQ for more details.
Cold or Off-line Backups - shut the database down and backup up ALL data, log, and control files.
Hot or On-line Backups - If the database is available and in ARCHIVELOG mode, set the tablespaces into backup mode and backup their files. Also remember to backup the control files and archived redo log files.
RMAN Backups - while the database is off-line or on-line, use the "rman" utility to backup the 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.

What is the difference between online and offline backups?

A hot (or on-line) backup is a backup performed while the database is open and available for use (read and write activity). Except for Oracle exports, one can only do on-line backups when the database is ARCHIVELOG mode.
A cold (or off-line) backup is a backup performed while the database is off-line and unavailable to its users. Cold backups can be taken regardless if the database is in ARCHIVELOG or NOARCHIVELOG mode.
It is easier to restore from off-line backups as no recovery (from archived logs) would be required to make the database consistent. Nevertheless, on-line backups are less disruptive and don't require database downtime.
Point-in-time recovery (regardless if you do on-line or off-line backups) is only available when the database is in ARCHIVELOG mode.

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.

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.

Issue the following commands to put a database into ARCHIVELOG mode:
SQL> CONNECT sys AS SYSDBA
SQL> STARTUP MOUNT EXCLUSIVE;
SQL> ALTER DATABASE ARCHIVELOG;
SQL> ARCHIVE LOG START;
SQL> ALTER DATABASE OPEN;

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

When a tablespace is in backup mode, Oracle will stop updating its file headers, but will continue to write to the data files.
When in backup mode, Oracle will write complete changed blocks to the redo log files. Normally only deltas (change vectors) 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.


How does one backup and restore a database using RMAN?

The biggest advantage of RMAN is that it 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/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 a 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.

Can one restore RMAN backups without a CONTROLFILE and RECOVERY CATALOG?

Details of RMAN backups are stored in the database control files and optionally a Recovery Catalog. If both these are gone, RMAN cannot restore the database. In such a situation one must extract a control file (or other files) from the backup pieces written out when the last backup was taken.