Database Reference
In-Depth Information
Types of RMAN Backups
There are a number of different types of backups in RMAN. First, we will discuss the
concept of consistent and inconsistent backups. This is an important fundamental building
block to understanding Oracle database backup and recovery operations, should they be
RMAN or manual.
Next we will discuss the two principle types of RMAN backups. The first type consists
of one or more backup sets. Backup sets are a very flexible way of backing up your Oracle
Database. The downside is that backup sets are not direct copies of Oracle Database data
files. Thus, you need RMAN to put the backup sets back together to restore your database.
Oracle also supports image copies . Image copies are direct copies of database data files.
Image copies offer faster recovery options but typically take up a great deal more space. In
the following sections, we will address these two types of RMAN backups in more detail.
Consistent and Inconsistent Backups
Perhaps the most important concept to understand with backups is the notion of a consis-
tent backup and an inconsistent backup. Oracle does not like anything to be out of order.
Because it's so persnickety about this consistency thing, Oracle checks all of the database
files, control files, and redo logs when you start the database. It makes sure that they are all
consistent to the same point in time. If it finds something amiss, then it will refuse to open
the database and the DBA has to figure out what the problem is.
This leads us to the concept of consistent and inconsistent database backups, which are
covered in the OCP exam. We will discuss concepts that are related to the idea of a consistent
database backup or inconsistent database backup through the next several chapters, so let's
take a moment to define what these terms mean.
A consistent database backup (sometimes called a cold backup since the database is not
running) is one that is taken when the database has been shut down in a normal manner
and where the state of the data in the database datafiles is consistent to the point in time
of the shutdown of the database. In other words, there is no redo to apply to the database
datafiles; it's all “in there.”
A consistent backup can be executed through RMAN, but it requires shutting down the
database first with the SHUTDOWN, SHUTDOWN IMMEDIATE, or SHUTDOWN
TRANSACTIONAL commands. In fact, if the database is in NOARCHIVELOG mode, it
will not allow you to perform an inconsistent database backup.
Once you have shut down the database normally, you will then mount the database and
issue the appropriate RMAN commands to start the database backup. Consistent database
backups are supported when Oracle is in ARCHIVELOG mode and NOARCHIVELOG
mode. However, if the database is in NOARCHIVELOG mode, it cannot perform point in
time recovery of the database. Instead, you will only be able to restore and recover the data-
base to the time of the actual backup. We will show you examples of inconsistent backups
and recoveries later in this and the next chapter.
At almost any point in the lifetime of a running Oracle database, the database datafiles
are inconsistent with reality. The newest changes are likely sitting in memory and wait-
ing to get written to disk. When a database is shut down normally, the graceful shutdown
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