先看一段官方文档的解释：

There is some CPU overhead associated with compressing backup sets. If the database being backed up is running at or near its maximum load, you may find the overhead from using AS COMPRESSED BACKUPSET unacceptable. In most other circumstances, compressing backupsets saves enough disk space to be worth the CPU overhead

总之、占用更多的 CPU 开销、但备份集减小了
测试一把（平台：10gR2 redhat）：

不使用压缩备份：

RMAN> list backup;

RMAN> list copy;

RMAN> backup database plus archivelog delete all input;

RMAN> list backup;

Size       Device     Elapsed Time  
------- ---- -- --- ------ -------- 
606.06M    DISK        00:01:26

使用压缩备份：

RMAN> list backup;

RMAN> list copy;

RMAN> backup as compressed backupset database plus archivelog delete all input;

RMAN> list backup;

Size       Device     Elapsed Time  
------- ---- -- --     ----- ------
112.27M    DISK        00:01:43

适用场景

在CPU并不是主要瓶颈的系统中、可以启动压缩备份
但库比较大的话、建议使用Image Copy Incremental Updates（ICIU）策略来解决

ITPub博客

RMAN Compressed Backupset

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