OCP 1Z0 053 149

149.You perform a backup using the following BACKUP command: 
RMAN> BACKUP AS COMPRESSED BACKUPSET DATABASE; 
Which statement is true of this command? 
A. A different procedure is required to restore a database from compressed backups 
B. The AS COMPRESSED clause of the BACKUP command provided by RMAN is used to create 
compressed backup sets and image copies. 
C. Using this command to create backups minimizes the bandwidth consumed 
D. Using this command to create backups improves the performance of the backup process 
Answer: C 

compressed只能应用于backupset，压缩会增加cpu


http://docs.oracle.com/cd/E11882_01/backup.112/e10642/rcmbckba.htm#BRADV8138

Making Compressed Backups

For any use of the BACKUP command that creates backup sets, you can take advantage of RMAN support for binary compression of backup sets. Specify the AS COMPRESSED BACKUPSET option to the BACKUP command.

RMAN compresses the backup set contents before writing them to disk. The details of which binary compression level is used are automatically recorded in the backup set. There is no need to explicitly mention the type of compression used or how to decompress the backup set in the recovery operation.

Example 9-7 backs up the entire database and archived logs to the configured default backup destination (disk or tape), producing compressed backup sets.

Example 9-7 Making Compressed Backups

BACKUP   AS COMPRESSED BACKUPSET   DATABASE PLUS ARCHIVELOG;

Binary compression creates some performance overhead during backup and restore operations. Binary compression consumes CPU resources, so compressed backups should not be scheduled when CPU usage is high. However, the following circumstances may warrant paying the performance penalty:

• You are using disk-based backups when disk space in your fast recovery area or other disk-based backup destination is limited.

• You are performing your backups to some device over a network when reduced network bandwidth is more important than CPU usage.

• You are using some archival backup media such as CD or DVD, where reducing backup sizes saves on media costs and archival storage.