FAQ about Backup & recovery

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.

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 doesn'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.

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.
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.

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


If Someone lost an archived/online REDO LOG file, How to restoreDB back?
The following INIT.ORA/SPFILE parameter can be used if your current redologs are corrupted or blown away. It may also be handy if you do database recovery and one of the archived log files are missing and cannot be restored.

_allow_resetlogs_corruption = true

This should allow you to open the database. However, after using this parameter your database will be inconsistent (some committed transactions may be lost or partially applied).
Steps:
--Do a "SHUTDOWN NORMAL" of the database
--Set the above parameter
--Do a "STARTUP MOUNT" and "ALTER DATATBASE OPEN RESETLOGS;"
--If the database asks for recovery, use an UNTIL CANCEL type recovery and apply all available archive and on-line redo logs, then issue CANCEL and reissue the "ALTER DATATBASE OPEN RESETLOGS;" command.
--Wait a couple of minutes for Oracle to sort itself out
--Do a "SHUTDOWN NORMAL"
--Remove the above parameter!
--Do a database "STARTUP" and check your ALERT.LOG file for errors.
--Extract the data and rebuild the entire database