Configuring and Backing Up Oracle Databases Using RMAN - OCP: Oracle Database 12c Administrator Certified Professional - page 143

We will discuss the internals of the recovery and restore process associated with incom-
plete recovery in later chapters in this topic, but for now just know that during a recovery
from an inconsistent backup, the DBA would restore the database datafiles they backed up,
and then simply apply the redo in the backed up redo logs. At the end of the restore, the
database is again consistent and ready to open.
RMAN Backup Sets
By default, when you create a backup in RMAN, it writes the backup to physical files. These
physical files are called backup set pieces (RMAN can also create backups called image copies,
which we will discuss later in this section). A given backup may create more than one backup
set piece. A collection of related backup set pieces is called a backup set . A backup set is a logi-
cal entity that is used to maintain the association of independent backup set pieces.
In addition to multiple backup set pieces, you may have more than one backup set. This
occurs when you parallelize a backup. Each channel will represent one backup set, each with
its own backup set pieces. New backup sets will also be created on a channel if a backup set
exceeds the backup set size limitations.
Note that a given data file can span backup set pieces but cannot span backup sets. The
ability of a given data file to span backup set pieces is known as multiplexing . Multiplexing
is another form of parallelization, since it allows RMAN to read from multiple data files
in parallel and write them to a single backup set piece. Thus, a given backup set piece may
have data from many data files in it.
In the default RMAN configuration, a given tablespace/data file backup may find itself
in more than one backup set piece. However, each individual tablespace/data file backup
can be associated with only a single RMAN backup set and thus will be backed up by only
one channel.
Oracle Database 12 c has a new feature called multisection backups. Multisection
backups allow you to parallelize the backup of large data files (bigfile tablespaces or
normal tablespaces). You may well find questions on multisection backups on your
OCP exam.
Figure 3.2 demonstrates the relationship between backups, backup sets, and backup
set pieces.
RMAN Image Copies
RMAN image copies are one-to-one copies of database data files. When you do an
image-copy backup of your database, you will receive no benefits of compression, so the
disk-space requirement is a one-to-one requirement. Image copies must be made to disk.
Figure 3.3 shows the difference between image and regular backups.
The upside to an image copy is that it can be much faster to restore. RMAN will always
choose to restore image copies over backup sets if an image copy is available. In fact, using
the SWITCH TO COPY command makes it even faster because RMAN will simply switch to the
image copy on disk and start using that copy (applying redo as required). To make an image
copy, use the BACKUP AS COPY command, as shown in this example:
RMAN> Backup as copy database;