Here is a scenario to understand Restore & Recovery
Sunday 10pm: Database is backed up. and is running fine.
Monday 11am: Went down / crashed due to some reason.
To bring up the database, we have 2 options:
1. Simple Restore: copying files from backup taken sunday night and open the database. Here, we loose all the changes that are done since sunday night.
2. Restore and Recovery: Copying files from backup taken sunday night and applying all the archivelog and redo log files to bring up the database to the point of failure. Here you dont loose the changes done until monday 11 am.
Restore: copying files from the backup overwriting the existing database files
Recovery: applying the changes to the database till point of failure. these changes are recorded in online redolog and archivelog (which are the backups of redolog) files.
A hot backup can be done by either RMAN,User Managed Backups by puting tablespace in backup mode my OEM which does the same as the user managed backup.But the Backup depends upon the size of the database you are using . if the database size in TB the RMAN backup will take more than 10 hours to complete and if the database is critical you can' wait for long to go for so long in this case their are special backup techniques which are given by vendors like TIVOLI and Netbackup they provide BC Vol backup called Business content Volumn Sync which copies a snapshot of the primary data to another place and backsup the database from one SAN to another with in 15 min for 2 TB of database and is the preferable method for big companies.
When physical disk fail, physical database file corrupt then media recovery required
Disk migration is noting but, migration of data from one OS dependent database to another Dependent database. The steps involved in this are
► first go to your target database and export all your data into flat files
► next in the destination database during the installation of the database, it asks for data source ,instead of giving the data of the oracle provided , give the path of the flat file you exported previously .
Database in archivelog mode chance to take hot backup and no data loss in recovery. you can use RMAN for backup and recovery .disadvantage is poor ferformance, and more chance to crash disc.