RMAN backup concept_ Online backup and backup mode

You can create RMAN Backup or user managed backup .

When performing a user managed backup of an online tablespace or database , Operating system tools can write processes in the database （DBWR） Back up data files while updating files . The tool may read a block in a semi - updated state , Half of the blocks copied to the backup media are updated , The other half contains old data . This type of logic corruption is called Fault block （fractured block）, That is, block and SCN atypism . If this backup must be restored , Block requires recovery , The recovery will fail because the block is unavailable .

For third-party snapshot technology , One of the following techniques must be used to eliminate the risk of creating broken blocks ：
1） Ensure that snapshot technology complies with Oracle Online backup requirements for
2） Take the database or data file offline
3） Put the database in backup mode before using a third-party snapshot backup .

Unlike user management tools ,RMAN No additional logging or backup mode is required , Because it knows the format of the data block .RMAN Ensure that broken blocks are not backed up . stay RMAN During the backup process , The database server session reads each data block , Check whether the block is a broken block by comparing its head and tail . If the block is broken , Then the session re reads the block . If the same fracture is found , Then the block is considered permanently damaged .RMAN There is no need to freeze the header checkpoint of the data file , Because it knows the read order of the blocks , Let it capture a known good checkpoint of the file .

source ：《Oracle Database Backup and Recovery User’s Guide,19c》