Databases Reference
In-Depth Information
Since RMAN image copies are identical to copies made with operating system copy commands, you may use
user-made image copies for an RMAN restore and recovery operation after first making the copies “known” to RMAN by
using the
catalog
command, as shown in Recipe 6-7. After this point, there's no difference between those image copies
made by you and those made by RMAN. During a restore operation, if you have both image copies and backup sets from
the same time period, RMAN prefers to use an image copy over a backup set. This is because there is more overhead
involved in sorting through a backup set to get the files to restore. In addition, image copies offer yet another benefit
during a restore and recovery operation. If you need to restore a current data file and happen to have an image copy of that
data file available, you can use the
switch
command to simply point the database to the replacement file instead of the
original data file. This eliminates the need to restore the data file, thus speeding up database recovery considerably.
RMAN Backup Modes
A control file or an archived redo log file is always backed up completely and in a consistent fashion. A data file,
however, may be backed up partly or completely. You can also make consistent or inconsistent backups with data
files. The various backup types are as follows:
Full vs. incremental backups:
A full backup is a backup of a data file that includes every
allocated block in that file. Note that an image copy backup of a data file will always include
every block in that file. A backup of a data file as a backup set, however, may skip data
blocks that aren't in use. An incremental backup can be one of two levels: a level 0 backup
including all blocks in the data file except those blocks compressed because they have
never been used, or a level 1 backup including only those blocks that have changed since
the parent backup
.
Consistent vs. inconsistent backups:
A backup taken after a database was shut down
gracefully (as opposed to using the
shutdown abort
command or a shutdown following an
abrupt database crash) and restarted in mount state is said to be consistent. A consistent
backup doesn't require recovery after you restore the database. A backup taken while the
database is online or after it was brought into mount state after being shut down abruptly is
called an inconsistent backup. An inconsistent backup always needs recovery to make the
backup consistent.
If you're running in archivelog mode, the target database must be mounted or be open before you can issue an
RMAN
backup
command. If you're running the database in noarchivelog mode, the database must first be shut down
cleanly and started up in mount state before you can use RMAN for backups. If the database was abruptly shut down and
restarted, RMAN can't make the backups. You mustn't back up a database running in noarchivelog mode while the
database is open.
■
starting with the Oracle Database 11
g
release, RMAN excludes the backup of undo in the undo tablespace,
which is not necessary for recovering an RMAN backup. unlike the backup optimization feature, you have no control over
whether to use this feature—it works by default. You can, if necessary, disable the uNDO BACkup OptiMiZAtiON feature
by setting the hidden parameter _
undo_block_compression
to
FALSE
.
Note
By default, all RMAN backups—whole database, tablespace level, and so on—are full backups. That is, all data
blocks in the data files that were ever used, even if they are currently empty, are included in the backup. You can
specify the command
backup full database
, for example, to start a whole-database backup, but it's not necessary
to do so. Just use the command
backup database
to do the same thing. However, when you are performing an
incremental RMAN backup, you must specify the keyword
incremental
in your backup commands since it isn't the
default backup type.
