网友在OTN中文官方技术论坛上提问问题：
"Exadata在rman备份时候的offloading功能需要数据库打开BCT吗？同题目，BCT=Block Change Tracking。
oracle数据库中BCT是使用文件来记录一组数据块中，修改过的数据块做个标记。
rman备份时，exadata的 offloading是如何实现的呢？"
As Maclean answered
"exadata的 offload incremental backup optimization 是基于数据块为单位， 而block change tracking通过位图维护一组block， 所以 offload incremental backup的 粒度更细化 也更智能。
With Exadata，changes are tracked at the individual oracle block level rather than at the level of a large group of blocks. This result in less I/O bandwidth being consumed for backups and faster running backups."
在Exadata上测试offload incremantal backup，未开启block change tracking的状态下， backup incremental level 1 200GB的数据文件，仅用7秒：
RMAN> report schema;
using target database control file instead of recovery catalog
Report of database schema for database with db_unique_name DBM
List of Permanent Datafiles
===========================
File Size(MB) Tablespace RB segs Datafile Name
---- -------- -------------------- ------- ------------------------
1 16384 SYSTEM *** +DATA_DM01/dbm/datafile/system.256.790880171
2 16384 SYSAUX *** +DATA_DM01/dbm/datafile/sysaux.257.790880183
3 16384 UNDOTBS1 *** +DATA_DM01/dbm/datafile/undotbs1.258.790880195
4 16384 UNDOTBS2 *** +DATA_DM01/dbm/datafile/undotbs2.260.790880213
5 1024 USERS *** +DATA_DM01/dbm/datafile/users.261.790880225
6 204800 TESTEXA *** +DATA_DM01/dbm/datafile/testexa.264.792624955
List of Temporary Files
=======================
File Size(MB) Tablespace Maxsize(MB) Tempfile Name
---- -------- -------------------- ----------- --------------------
1 32767 TEMP 33554431 +DATA_DM01/dbm/tempfile/temp.259.790880685
RMAN>
RMAN> backup as compressed backupset incremental level 1 tablespace testexa;
Starting backup at 05-SEP-12
allocated channel: ORA_DISK_1
channel ORA_DISK_1: SID=784 instance=dbm1 device type=DISK
allocated channel: ORA_DISK_2
channel ORA_DISK_2: SID=850 instance=dbm1 device type=DISK
allocated channel: ORA_DISK_3
channel ORA_DISK_3: SID=914 instance=dbm1 device type=DISK
allocated channel: ORA_DISK_4
channel ORA_DISK_4: SID=981 instance=dbm1 device type=DISK
allocated channel: ORA_DISK_5
channel ORA_DISK_5: SID=1042 instance=dbm1 device type=DISK
allocated channel: ORA_DISK_6
channel ORA_DISK_6: SID=1109 instance=dbm1 device type=DISK
allocated channel: ORA_DISK_7
channel ORA_DISK_7: SID=1174 instance=dbm1 device type=DISK
allocated channel: ORA_DISK_8
channel ORA_DISK_8: SID=1238 instance=dbm1 device type=DISK
allocated channel: ORA_DISK_9
channel ORA_DISK_9: SID=1302 instance=dbm1 device type=DISK
allocated channel: ORA_DISK_10
channel ORA_DISK_10: SID=1369 instance=dbm1 device type=DISK
allocated channel: ORA_DISK_11
channel ORA_DISK_11: SID=1435 instance=dbm1 device type=DISK
allocated channel: ORA_DISK_12
channel ORA_DISK_12: SID=3 instance=dbm1 device type=DISK
channel ORA_DISK_1: starting compressed incremental level 1 datafile backup set
channel ORA_DISK_1: specifying datafile(s) in backup set
input datafile file number=00006 name=+DATA_DM01/dbm/datafile/testexa.264.792624955
channel ORA_DISK_1: starting piece 1 at 05-SEP-12
channel ORA_DISK_1: finished piece 1 at 05-SEP-12
piece handle=+RECO_DM01/dbm/backupset/2012_09_05/nnndn1_tag20120905t100651_0.499.793188411 tag=TAG20120905T100651 comment=NONE
channel ORA_DISK_1: backup set complete, elapsed time: 00:00:07
Finished backup at 05-SEP-12