我的是10g，做rman的备份测试：

开始：
CONFIGURE DEVICE TYPE DISK PARALLELISM 1
然后
backup
  format '/tool/rmanbackup/db_%d_%s_%p.bus'
  database filesperset=2;

得到3个备份集。为16,17,18：可以看出每2个数据文件生产一个备份集。
List of Backup Sets
===================

BS Key  Type LV Size       Device Type Elapsed Time Completion Time   
------- ---- -- ---------- ----------- ------------ -------------------
16      Full    357.78M    DISK        00:00:12     2010-05-14 12:28:16
        BP Key: 16   Status: AVAILABLE  Compressed: NO  Tag: TAG20100514T122804
        Piece Name: /tool/rmanbackup/db__18_1.bus
  List of Datafiles in backup set 16
  File LV Type Ckp SCN    Ckp Time            Name
  ---- -- ---- ---------- ------------------- ----
  1       Full 1456934    2010-05-14 12:28:04 /tool/oracle10g/oradata/system01.dbf
  2       Full 1456934    2010-05-14 12:28:04 /tool/oracle10g/oradata/undotbs01.dbf

BS Key  Type LV Size       Device Type Elapsed Time Completion Time   
------- ---- -- ---------- ----------- ------------ -------------------
17      Full    196.43M    DISK        00:00:08     2010-05-14 12:28:27
        BP Key: 17   Status: AVAILABLE  Compressed: NO  Tag: TAG20100514T122804
        Piece Name: /tool/rmanbackup/db__19_1.bus
  List of Datafiles in backup set 17
  File LV Type Ckp SCN    Ckp Time            Name
  ---- -- ---- ---------- ------------------- ----
  3       Full 1456945    2010-05-14 12:28:19 /tool/oracle10g/oradata/sysaux01.dbf
  4       Full 1456945    2010-05-14 12:28:19 /tool/oracle10g/oradata/users01.dbf

BS Key  Type LV Size       Device Type Elapsed Time Completion Time   
------- ---- -- ---------- ----------- ------------ -------------------
18      Full    5.45M      DISK        00:00:02     2010-05-14 12:28:36
        BP Key: 18   Status: AVAILABLE  Compressed: NO  Tag: TAG20100514T122804
        Piece Name: /tool/rmanbackup/db__20_1.bus
  Control File Included: Ckp SCN: 1456954      Ckp time: 2010-05-14 12:28:34
  SPFILE Included: Modification time: 2010-05-14 11:02:52

当我设置了CONFIGURE DEVICE TYPE DISK PARALLELISM 3，再同样运行：生成19,20,21,22共4个，这是为啥啊？为啥把controlfile和spcile分开了呢？
BS Key  Type LV Size       Device Type Elapsed Time Completion Time   
------- ---- -- ---------- ----------- ------------ -------------------
19      Full    5.42M      DISK        00:00:00     2010-05-14 12:43:40
        BP Key: 19   Status: AVAILABLE  Compressed: NO  Tag: TAG20100514T124339
        Piece Name: /tool/rmanbackup/db_JS10G_23_1.bus
  Control File Included: Ckp SCN: 1457443      Ckp time: 2010-05-14 12:43:40

BS Key  Type LV Size       Device Type Elapsed Time Completion Time   
------- ---- -- ---------- ----------- ------------ -------------------
20      Full    80.00K     DISK        00:00:00     2010-05-14 12:43:41
        BP Key: 20   Status: AVAILABLE  Compressed: NO  Tag: TAG20100514T124339
        Piece Name: /app/tool/rmanbackup/db_JS10G_24_1.bus
  SPFILE Included: Modification time: 2010-05-14 11:02:52

BS Key  Type LV Size       Device Type Elapsed Time Completion Time   
------- ---- -- ---------- ----------- ------------ -------------------
21      Full    196.43M    DISK        00:00:09     2010-05-14 12:43:49
        BP Key: 21   Status: AVAILABLE  Compressed: NO  Tag: TAG20100514T124339
        Piece Name: /app/tool/rmanbackup/db_JS10G_22_1.bus
  List of Datafiles in backup set 21
  File LV Type Ckp SCN    Ckp Time            Name
  ---- -- ---- ---------- ------------------- ----
  3       Full 1457444    2010-05-14 12:43:40 /tool/oracle10g/oradata/sysaux01.dbf
  4       Full 1457444    2010-05-14 12:43:40 /tool/oracle10g/oradata/users01.dbf

BS Key  Type LV Size       Device Type Elapsed Time Completion Time   
------- ---- -- ---------- ----------- ------------ -------------------
22      Full    357.78M    DISK        00:00:17     2010-05-14 12:43:57
        BP Key: 22   Status: AVAILABLE  Compressed: NO  Tag: TAG20100514T124339
        Piece Name: /app/tool/rmanbackup/db_JS10G_21_1.bus
  List of Datafiles in backup set 22
  File LV Type Ckp SCN    Ckp Time            Name
  ---- -- ---- ---------- ------------------- ----
  1       Full 1457445    2010-05-14 12:43:40 /tool/oracle10g/oradata/system01.dbf
  2       Full 1457445    2010-05-14 12:43:40 /tool/oracle10g/oradata/undotbs01.dbf


并行度和filesperset 有啥关系呢？能详细举例说明下么？