Oracle RMAN tutorial: Get started with database backup

Creating database backups

Not surprisingly, the command used to create an RMAN backup is BACKUP. To back up a full database, for example, you would use this command:

BACKUP DATABASE;

To back up the database and the corresponding archive logs, you would use:

BACKUP DATABASE PLUS ARCHIVELOG;

The BACKUP command is far less limiting than these RMAN tutorial examples might lead one to believe, and it even includes a wide variety of helpful options.

One such option is to use the TAG parameter. If you use RMAN to create backups on a regular basis, you will need a way to distinguish between the backups. This is where backup tags come into play.

If you use the LIST command to display a list of the current backups, each backup is assigned a tag. However, you can apply a custom tag to a backup instead of the system-generated tag.

The simplest way to create a backup is to use the BACKUP DATABASE; command. With that in mind, if I want to create a tag containing my name (Brien) as a way of showing that I was the one who created the backup, I would use this command:

BACKUP DATABASE TAG = 'Brien';

Compressed backup

RMAN enables you to create compressed backups to reduce the amount of space consumed by the backup. The degree to which compression can shrink a backup varies widely depending on the data within the database, but it is common to achieve a reduction rate of 70% to 80%.

The command used to create a compressed backup is simple, but it's important to understand the concept of a backup set.

So far in this RMAN tutorial, there have been several examples of how to use the BACKUP command to create a backup. To be technically precise, however, what the BACKUP command creates is a backup set, which includes data from one or more data files, any archived redo logs, control files or server parameter files.

When you compress a backup, you are actually compressing the backup set. The command even mentions the backup set. If you want to run the BACKUP DATABASE; command, but want the resulting backup to be compressed, the command would be:

BACKUP AS COMPRESSED BACKUPSET DATABASE;

Incremental backup

RMAN can also do incremental backups. However, this process might work a little differently than what you would expect, so we'll take a closer look at the process in this RMAN tutorial.

Incremental backups usually refer to a backup of everything that has changed since the last full backup. Even though the BACKUP DATABASE; command creates a full backup, the resulting backup is not compatible with incremental backups.

If you plan on making incremental backups, the full backup has to be taken as a level 0 backup. The level 0 backup is just another type of full backup, but one that can be used as the basis for incremental backups. You can create a level 0 backup by using this command:

BACKUP INCREMENTAL LEVEL 0 DATABASE;

Once a level 0 backup exists, you can create incremental backups, which are differential or cumulative. Differential backups are the default, and are created using a command that is almost identical to the one used to create a level 0 backup. The only difference is that you will need to create a level 1 backup instead. The command is:

BACKUP INCREMENTAL LEVEL 1 DATABASE;

If you want the backup to be cumulative, then the command would be:

BACKUP INCREMENTAL LEVEL 1 CUMULATIVE DATABASE;

When you create an incremental backup, that backup is defined as a level 1 backup. A level 1 backup has to have a parent backup, and that parent can be a level 0 backup or a previous level 1 backup.

You can learn more about what the RMAN BACKUP command can do by checking out the Oracle documentation.