1、RMAN备份默认已经进行压缩
1）不用的块压缩
When employing unused block compression, RMAN skips reading, and backing up, any database blocks that are not currently allocated to some database object. This is regardless of whether those blocks had previously been allocated.

So if a database table is dropped, RMAN will not back up the space that was occupied by that table until new objects are created in that space.

Unused block compression is used automatically when the following conditions are true:

The COMPATIBLE initialization parameter is set to 10.2 or higher.

There are currently no guaranteed restore points defined for the database.

The data file is locally managed.

The data file is being backed up to a backup set as part of a full backup or a level 0 incremental backup.

The backup set is created on disk, or Oracle Secure Backup is the media manager.

2）空块压缩
When employing null block compression, RMAN omits from its output any block that has never contained data.

Null block compression is always used with level 0 or full backups that are created in backup set format.

小结：

1）这样我们可以解释了，为什么全备份输入的大小和输出的大小不一致了。
2）归档日志输入输出大小一致，说明不会做压缩，只有全归档日志备份。

START_TIME       INPUT_TYPE    STATUS     INPUT_SIZE   DEVICE_TYP OUTPUT_SIZE  TIME_TAKEN IN_SPEED   OUT_SPEED
---------------- ------------- ---------- ------------ ---------- ------------ ---------- ---------- ----------
2018-10-04 12:01 ARCHIVELOG    COMPLETED      2.58G    SBT_TAPE       2.58G    00:03:06      14.20M     14.20M
2018-10-04 13:00 ARCHIVELOG    COMPLETED      2.50G    SBT_TAPE       2.50G    00:03:24      12.54M     12.55M
2018-10-04 18:01 ARCHIVELOG    COMPLETED      2.17G    SBT_TAPE       2.17G    00:03:15      11.39M     11.39M
2018-10-04 19:01 ARCHIVELOG    COMPLETED      2.11G    SBT_TAPE       2.11G    00:03:29      10.33M     10.34M
2018-10-05 01:01 DB FULL       COMPLETED    686.17G    SBT_TAPE     565.61G    00:37:05     315.79M    260.31M
2018-10-05 06:00 ARCHIVELOG    COMPLETED      1.50G    SBT_TAPE       1.51G    00:03:14       7.94M      7.95M
2018-10-05 07:01 ARCHIVELOG    COMPLETED    992.30M    SBT_TAPE     993.00M    00:02:59       5.54M      5.55M
2018-10-05 12:01 ARCHIVELOG    COMPLETED      2.41G    SBT_TAPE       2.41G    00:03:14      12.73M     12.74M