On the other hand, differential backups only record the
changes since the last backup, so each backupset might be smaller or larger than the previous
one, with no overlap in data blocks backed up. However, a restore and recover operation may
take longer if you have to restore from several backupsets instead of just two.
Following our example with the dw database, we find out that several files are outside of our
retention policy of four days; in other words, files that need more than four days worth of archived
redo logs to recover the database:
RMAN> report need backup;
RMAN retention policy will be applied to the command
RMAN retention policy is set to recovery window of 4 days
Report of files whose recovery needs more than 4 days of archived logs
446 Oracle Database 11g DBA Handbook
File Days Name
---- ----- -----------------------------------------------------
1 5 +DATA/dw/datafile/system.256.630244579
2 5 +DATA/dw/datafile/sysaux.257.630244581
3 5 +DATA/dw/datafile/undotbs1.258.630244583
4 5 +DATA/dw/datafile/users.259.630244583
5 5 +DATA/dw/datafile/example.265.630244801
6 5 +DATA/dw/datafile/users_crypt.267.630456963
7 5 +DATA/dw/datafile/inet_star.268.632004213
8 5 +DATA/dw/datafile/inet_intl_star.269.632009933
RMAN>
To remedy this situation, we can do another full backup, or we can pursue an incremental
backup policy, which might be easier to implement and maintain. To set up our incremental
policy, we need to perform a level 0 incremental backup first:
RMAN> backup incremental level 0
2> as compressed backupset database;
Starting backup at 30-AUG-07
using channel ORA_DISK_1
channel ORA_DISK_1: starting compressed incremental level 0
datafile backup set
channel ORA_DISK_1: specifying datafile(s) in backup set
input datafile file number=00002 name=+DATA/dw/datafile/sysaux.
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