在Oracle10g就已经推出了Compressed Backupset。

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.

今天在自己机器上的Oracle 11.1.0.6版本中测试了一下，压缩效果还是很让人满意的，不但存储空间变小，甚至备份所需要的时间也有所加快。

不使用压缩的备份：
backup database plus archivelog delete all input;

使用压缩的备份：
backup AS COMPRESSED BACKUPSET database plus archivelog delete all input;

在CPU并不是主要瓶颈的系统中，建议启动压缩备份。

Update：
Oracle11g中新增的zlib压缩算法和之前bzip2压缩算法之间的测试比较可以参看Oracle11g新特性：RMAN压缩备份 from Ningoo 。