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Lesson 3 Listing the different methods of recovery
Objective Explain how the recovery depends on the state of the database.

Listing the Different Recovery Methods in Oracle

When you perform a complete recovery, you need to choose a recovery method based on the state of your database, and this depends on which files are damaged. There are four different methods for complete recovery. The following table describes the situations when a specific method should be applied.

RECOVERY METHOD FILES NEEDING RECOVERY RECOMMENDED USE
Closed Database The damaged files belong to the SYSTEM or a rollback segment tablespace The whole database, or a majority of the datafiles, if damaged The database does not have to be running at all times--24 hours per day, 7 days per week.
Open Database, initially open Damaged files that do not belong to the SYSTEM or rollback segment tablespaces The database is a 24x7, and you have to keep the down-time of the database to a minimum While the database is running, a media failure or a file corruption occurs, or there is an accidental loss of file/s
Open Database, initially closed The damaged files do not belong to the SYSTEM or rollback segment tablespaces A media failure or a hardware crash has shut down the system The database is 24 and 7, and you have to keep the down-time of the database to a minimum
Recovering a datafile with no backup (This method is covered in a later module.) The damaged files do not belong to the SYSTEM or rollback segment tablespaces A media failure or user failure occurs and results in the loss of a datafile that was never backed up or is too new to have been picked up by a scheduled backup All archived logs are available since the file was created

During recovery, all the required archived log files must be located on a disk that Oracle can access. If you store these files on a tape to save disk space, you must restore these files to the disk before recovery starts.


Types of Backup and Recovery Options

There are two major categories of backup:
  1. Full backup: Includes backups of datafiles, datafile copies, tablespaces, control files (current or backup), or the entire database (including all datafiles and the current control file). Reads entire files and copies all blocks into the backup set, skipping only datafile blocks that have never been used (with the exception of control files and redo logs where no blocks are skipped).
  2. Incremental backup: Includes backups of datafiles, tablespaces, or the whole database. Reads entire files and backs up only those data blocks that have changed since a previous backup.
You can begin backups through the Recovery Manager or the Oracle Enterprise Manager interface to RMAN, which uses the database export facility, or you can initiate backups via standard operating system backup utilities. In general, RMAN supports most database backup features, including
  1. open or online backups,
  2. closed database backups,
  3. incremental backups at the Oracle block level,
  4. corrupt block detection,
  5. automatic backups,
  6. backup catalogs, and
  7. backups to sequential media.
RMAN added capabilities in Oracle9i for one-time backup configuration, recovery windows to determine and manage expiration dates of backups, and restartable backups and restores. Also added was support for testing of restores and recovery.
Since Oracle Database 10g, RMAN can perform image copy backups of the database, tablespaces, or datafiles. RMAN can be used to apply incremental backups to datafile image backups. The speed of incremental backups is increased through a change-tracking feature by reading and backing up only changed blocks.
The next lesson describes the complete recovery operation.