RMAN Backups and Reporting - Oracle RMAN for Absolute Beginners - page 123

The last task RMAN does as part of the backup is to generate a backup set that contains a backup of the control
file. This control file will contain all information regarding the backup that took place and any archive redo logs that
were generated during the backup.

■ Tip always enable the autobackup of the control file feature.

There are many nuances to the RMAN BACKUP command. For production databases, I usually back up the
database with the BACKUP INCREMENTAL LEVEL=0 DATABASE PLUS ARCHIVELOG command. That's generally sufficient.
However, you will encounter many situations in which you need to run a backup that uses a specific RMAN feature,
or you might troubleshoot an issue requiring that you be aware of the other ways to invoke an RMAN backup. These
aspects are discussed in the next several sections.

Full Backup vs. Incremental Level=0
The term RMAN full backup sometimes causes confusion. A more apt way of phrasing this task would be RMAN
backing up all modified blocks within one or more data files . The term full does not mean that all blocks are backed up
or that all data files are backed up. It simply means that all blocks that would be required to rebuild a data file (in the
event of a failure) are being backed up. You can take a full backup of a single data file, and the contents of that backup
piece may be quite a bit smaller than the data file itself.
The term RMAN level 0 incremental backup doesn't exactly describe itself very well, either. A level 0 incremental
backup is backing up the same blocks as a full backup. In other words, the following two commands back up the same
blocks in a database:

RMAN> backup as backupset full database;
RMAN> backup as backupset incremental level=0 database;

The only difference between the prior two commands is that an incremental level 0 backup can be used in
conjunction with other incremental backups, whereas a full backup cannot participate in an incremental backup
strategy. Therefore, I almost always prefer to use the INCREMENTAL LEVEL=0 syntax (as opposed to a full backup); it
gives me the flexibility to use the level 0 incremental backup along with subsequent incremental level 1 backups.

Backup Sets vs. Image Copies
The default backup mode of RMAN instructs it to back up only blocks that have been used in a data file; these are
known as backup sets. RMAN can also make byte-for-byte copies of the data files; these are known as image copies.
Creating a backup set is the default type of backup that RMAN creates. The next command creates a backup set
backup of the database:

RMAN> backup database;

If you prefer, you can explicitly place the AS BACKUPSET command when creating backups:

RMAN> backup as backupset database;

You can instruct RMAN to create image copies by using the AS COPY command. This command creates image
copies of every data file in the database:

RMAN> backup as copy database;