621.You have enabled backup optimization for the RMAN environment. Identify two criteria on which
RMAN will skip the file, if it has already been backed up. (Choose two.)
A. The data file backup is done with multiple channels
B. The data files is in the read-write mode after being backed up in the read only mode
C. The backup was taken after the data files was taken offline-normal or is in the read only mode
D. The data file backup complies with the back retention policy and the backup duplexing feture
Answer: CD
答案解析：
参考1：http://docs.oracle.com/cd/E11882_01/backup.112/e10642/rcmbckad.htm#BRADV89548
Example 10-1 Configuring Backup Optimization
With RMAN configured as shown in Example 10-1, you run the following command every night to back up the database to tape:
Because backup optimization is configured, RMAN skips backups of offline and read-only data files only if the most recent backups were made on or after the earliest point in the recovery window. RMAN does not skip backups when the most recent backups are older than the window. For example, optimization ensures you do not end up with a new backup of a read-only data file every night, so long as one backup set containing this file exists within the recovery window.
参考：http://docs.oracle.com/cd/E11882_01/backup.112/e10643/rcmsynta010.htm#RCMRF90093
BACKUP OPTIMIZATION[ON | OFF | CLEAR] |
Toggles backup optimization ON or OFF (default). Specify CLEAR to return optimization to its default value of OFF.
Backup optimization is enabled when all of the following conditions are met:
Optimization prevents RMAN from backing up a file to a device type if the identical file is already backed up on the device type. RMAN does not signal an error if backup optimization causes all files to be skipped during a backup. The backup retention policy affects which files backup optimization skips. For two files to be identical, their content must be exactly the same, as described in Table 2-3. When you create backup pieces on disk or on media managed by Oracle Secure Backup, optimization excludes undo data from the backup when the data does not belong to an active transaction. Note: Note: You can override backup optimization with the See Also: Oracle Database Backup and Recovery User's Guide for a description of how RMAN determines that it can skip the backup of a file |
Example 2-48 Optimizing Backups
This scenario illustrates the backup optimization behavior described in Table 2-3. Assume that backup optimization is disabled. At 9 a.m., you back up three copies of all existing archived redo log files to tape.
The BACKUP_TAPE_IO_SLAVES initialization parameter must be true when duplexing backups to tape.
At 11 a.m., you enable backup optimization:
At noon, you run the following archived redo log backup:
In this case, the BACKUP ... COPIES setting overrides the CONFIGURE ... COPIES setting, so RMAN sets n=2. RMAN skips the backup
of a log only if at least two copies of the log exist on the sbt device. Because three copies of each log exist on sbt of all the logs generated on or before 9 a.m., RMAN skips the backups of these logs. However, RMAN backs up two
copies of all logs generated after 9 a.m. because these logs have not yet been backed up to tape.