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1) We know that at the start of backup rman needs to resysnc recovery catalog with current control file - so a snapshot control is created before initiating full backup of the database?
I think snapshot controlfile is not created before initiating the backup because RMAN synchronize the recovery catalog with controlfile at the end of backup operation so that latest backup information also gets update to the recovery catalog.
2) Now in the end once when the whole database backup & archivelog backup is complete , while taking current controlfile backup as per written script, does it again create a new snapshot controlfile or just the snapshot controlfile is accessed and does the snapshot controlfile remains in locked state(read only) during entire backup operation ?
Yes snapshot controlfile is created while taking controlfile backup.
RMAN makes a copy of the controlfile to read the physical structure of the database. It's an intermediate copy of the control file for integrity reasons. A snapshot control file is created when RMAN synchronizes the backups recorded in the control file with the RMAN catalog or when backing up the control file. Keep in mind that RMAN always updates the current controlfile with information about the backupsets and backuppieces created.
Control file autobackup creates a backup control file every time the database structure changes, e.g. adding datafile, making datafile read-only, etc.
The first time it creates a snapshot, it uses the snapshot to update the catalog.
It then runs the backup of the database. The BACKUP DATABASE command has to update the current controlfile with information about each backupset and backuppiece created and information about the datafiles in each backuppiece. Therefore, the current controlfile has more information than the snapshot created first.
Then, for the BACKUP CURRENT CONTROLFILE, it has to take a new snapshot of the updated current controlfile.
Hemant K Chitale