During begin backup mode datafile headers get freezed and as a result row information cannot be retrieved as a result the entire block is copied to redo logs as a result more redo generated and more log switch and in turn more archive logs.
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.
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 .
When physical disk fail, physical database file corrupt then media recovery required
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.
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.
To Enable Restricted Session
Alter system enable restricted session;
To Disable Restricted Session
Alter system disable restricted session;
To Start the Database in Restricted Mode
By starting Instance in restricted mode it will not allow all users to access and only users with restriction privilege will be allowed to access.
This will be done time of make some data changes so that no users should be allowed to access data on time of changes happening
8. What is db_recovery_file_dest in oracle? When do you need to set this value?
Give me the steps to perform the point in time recovery with a backup which is taken before the reset logs of the db?
Tell me about the steps required to enable the RMAN backup for a target database?
In oracle db_recovery_file_dest specifies a default location of flash recovery area which contains multiplexed current control files,
online redo logs as well as archived logs, Rman backups,flash back logs.
db_recovery_file_dest_size should be specified as well.
Media recovery is a process to recover database from backup when physical disk failure occure.
cash recovery is a automated process take care by oracle when instance failure occure.