Full
Backups
A full
backup is different from a whole database backup. A full data file backup is a
backup that includes every used data block in the file. RMAN copies all blocks
into the backup set or image copy, skipping only those data file blocks that
have never been used. For a full image copy, the entire file contents are
reproduced exactly. A full backup cannot be part of an incremental backup
strategy; it cannot be the parent for a subsequent incremental backup.
Incremental
Backups
An
incremental backup is either a level 0 backup, which includes every block in
the data files except blocks that have never been used, or a level 1 backup,
which includes only those blocks that have been changed since a previous backup
was taken. A level 0 incremental backup is physically identical to a full
backup. The only difference is that the level 0 backup can be used as the base
for a level 1 backup, but a full backup can never be used as the base for a
level 1 backup.
Incremental
backups are specified using the INCREMENTAL keyword of the BACKUP command. You
specify INCREMENTAL LEVEL [0 | 1].
RMAN can
create multilevel incremental backups as follows:
•
Differential: Is the default type of incremental
backup that backs up all blocks changed after the most recent incremental
backup at either level 1 or level 0
•
Cumulative: Backs up all blocks changed after
the most recent backup at level 0
Examples
•
To
perform an incremental backup at level 0, use the following command:
RMAN> BACKUP INCREMENTAL LEVEL 0 DATABASE;
•
To
perform a differential incremental backup, use the following command:
RMAN> BACKUP INCREMENTAL LEVEL 1 DATABASE;
•
To
perform a cumulative incremental backup, use the following command:
RMAN> BACKUP INCREMENTAL LEVEL 1 CUMULATIVE DATABASE;
RMAN makes
full backups by default if neither FULL nor INCREMENTAL is specified. Unused
block compression causes never-written blocks to be skipped when backing up
data files to backup sets, even for full backups.
A full
backup has no effect on subsequent incremental backups, and is not considered
part of any incremental backup strategy, although a full image copy backup can
be incrementally updated by applying incremental backups with the RECOVER
command.
Fast
Incremental Backup
The goal of
an incremental backup is to back up only those data blocks that have changed
since a previous backup. You can use RMAN to create incremental backups of data
files, tablespaces, or the whole database. When making an incremental backup,
RMAN scans each block of the data files to see which have changed since the
last backup. That makes the backup smaller because only changed blocks are backed
up. It also makes recovery faster because fewer blocks need to be restored.
You can
perform fast incremental backup by enabling block change tracking. Block change
tracking writes to a file the physical address of each block that is changed.
When it is time to perform the incremental backup, RMAN can look at the block
change tracking file, and back up only those blocks referenced there; it does
not have to scan every block to see if it has changed since the last backup.
This makes the incremental backup faster.
The
maintenance of the tracking file is fully automatic and does not require your
intervention. The size of the block change tracking file is proportional to
the:
•
Database
size, in bytes
•
Number
of enabled threads in a RAC environment
•
Number
of old backups maintained by the block change tracking file
•
The minimum
size for the block change tracking file is 10 MB, and any new space is
allocated in 10 MB increments. The Oracle database does not record block change
information by default.
Enabling
Fast Incremental Backup
You enable
block change tracking from the Database Control home page. Navigate to
Availability > Backup Settings > Policy. You do not need to set the block
change tracking file destination if the DB_CREATE_FILE_DEST initialization parameter
is set because the file is created as an Oracle Managed File (OMF) file in the
DB_CREATE_FILE_DEST location. You can, however, specify the name of the block
change tracking file, placing it in any location you choose.
You can
also enable or disable this feature by using an ALTER DATABASE command. If the
change tracking file is stored in the database area with your database files,
then it is deleted when you disable change tracking.
ALTER
DATABASE
{ENABLE|DISABLE} BLOCK CHANGE TRACKING
{ENABLE|DISABLE} BLOCK CHANGE TRACKING
[USING
FILE '...']
You can
rename the block change tracking file by using the ALTER DATABASE RENAME
command. Your database must be in the MOUNT state to rename the tracking file.
The ALTER DATABASE RENAME FILE command updates the control file to refer to the
new location. You can use the following syntax to change the location of the
block change tracking file:
ALTER DATABASE RENAME FILE
'...' TO '...';
