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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