Backup and Recovery Concepts
Log mode decides your options
In NOARCHIVELOG mode filled redo logs are overwritten, so the only valid backups are consistent ones taken after a clean shutdown. Recovery is restore-only and everything since the backup is lost. In ARCHIVELOG mode ARCn archives every filled log, so you may take inconsistent (online, open-database) backups and perform complete or point-in-time recovery.
RMAN vocabulary
- Backup set - RMAN's proprietary format, compressible and multiplexed; must be restored before use.
- Image copy - a byte-for-byte copy of a datafile, usable directly.
- Full backup - every used block of the datafiles it covers.
- Level 0 - the base of an incremental strategy.
- Level 1 differential - blocks changed since the most recent level 0 or level 1; cumulative - blocks changed since the most recent level 0 only.
Restore copies files back from backups; recover applies redo (and undo) to make them consistent. Instance recovery happens automatically after a crash when all files are intact; media recovery is required when a file is lost or damaged. The FRA (DB_RECOVERY_FILE_DEST) holds archived logs, backups and flashback logs by default. Retention policies mark backups obsolete; files RMAN cannot find are expired.
Traps: "restore" and "recover" are not interchangeable in exam answers; a level 0 backup must exist before level 1; a consistent backup needs no recovery at all.
Rule of thumb: ask two questions - is the database in ARCHIVELOG mode, and are all current files present? Archived plus a damaged file means media recovery; NOARCHIVELOG plus a damaged file means restore and accept data loss.