filesperset说明了一个备份集最多的文件数目，FILESPERSET不指定的话，FILESPERSET(默认值=64)
filesperset =files per backupset
假如10个datafiles,filesperset =4，10/4=2.5，你备份数据库的时候生成3个backupset
当指定filesperset参数时，rman比较filesperset与自动计算出来的值（对每个已分配通道的文件数目）,并取其中较小的那个值来保证所有的通道被使用。
如果没有指定filesperset，rman比较计算出来的值（文件数目除以已分配的通道）和默认值64，并取其中较小的那个值来保证所有通道可用。一个例外是通道比要备份的文件还要多
例如：
A. filesperset设置为6，数据文件数目为30，通道数据为4，通过30/4可以得出每个
备份集可含有8个文件，取6和8中较小的值6，那么30/6=5个备份集，那么4个通道肯定都有事情可做了。
B. 如果不指定filesperset，假设数据文件数目为30，通道数据为4，通过30/4可以
得出每个备份集可含有8个文件，比较8和默认值64，我们取其中较小的8，那么也可以保证4个通道都有事情可做
SQL> select file_name from dba_data_files where tablespace_name=‘USERS’;
FILE_NAME
/u01/app/oracle/oradata/PROD7/users01.dbf
/u01/app/oracle/oradata/PROD7/users02.dbf
/u01/app/oracle/oradata/PROD7/users03.dbf
/u01/app/oracle/oradata/PROD7/users04.dbf
RMAN> backup filesperset 3 tablespace users format ‘/u01/backup/users%U’;
Starting backup at 09-AUG-18
using target database control file instead of recovery catalog
allocated channel: ORA_DISK_1
channel ORA_DISK_1: SID=224 device type=DISK
channel ORA_DISK_1: starting full datafile backup set
channel ORA_DISK_1: specifying datafile(s) in backup set
input datafile file number=00007 name=/u01/app/oracle/oradata/PROD7/users03.dbf
input datafile file number=00008 name=/u01/app/oracle/oradata/PROD7/users04.dbf
channel ORA_DISK_1: starting piece 1 at 09-AUG-18
channel ORA_DISK_1: finished piece 1 at 09-AUG-18
piece handle=/u01/backup/users01ta4lfl_1_1 tag=TAG20180809T144925 comment=NONE
channel ORA_DISK_1: backup set complete, elapsed time: 00:00:01
channel ORA_DISK_1: starting full datafile backup set
channel ORA_DISK_1: specifying datafile(s) in backup set
input datafile file number=00006 name=/u01/app/oracle/oradata/PROD7/users02.dbf
input datafile file number=00004 name=/u01/app/oracle/oradata/PROD7/users01.dbf
channel ORA_DISK_1: starting piece 1 at 09-AUG-18
channel ORA_DISK_1: finished piece 1 at 09-AUG-18
piece handle=/u01/backup/users02ta4lfm_1_1 tag=TAG20180809T144925 comment=NONE
channel ORA_DISK_1: backup set complete, elapsed time: 00:00:01
Finished backup at 09-AUG-18
RMAN> backup filesperset 1 tablespace users format ‘/u01/backup/users%U’;
Starting backup at 09-AUG-18
using channel ORA_DISK_1
channel ORA_DISK_1: starting full datafile backup set
channel ORA_DISK_1: specifying datafile(s) in backup set
input datafile file number=00006 name=/u01/app/oracle/oradata/PROD7/users02.dbf
channel ORA_DISK_1: starting piece 1 at 09-AUG-18
channel ORA_DISK_1: finished piece 1 at 09-AUG-18
piece handle=/u01/backup/users03ta4lg4_1_1 tag=TAG20180809T144939 comment=NONE
channel ORA_DISK_1: backup set complete, elapsed time: 00:00:01
channel ORA_DISK_1: starting full datafile backup set
channel ORA_DISK_1: specifying datafile(s) in backup set
input datafile file number=00007 name=/u01/app/oracle/oradata/PROD7/users03.dbf
channel ORA_DISK_1: starting piece 1 at 09-AUG-18
channel ORA_DISK_1: finished piece 1 at 09-AUG-18
piece handle=/u01/backup/users04ta4lg5_1_1 tag=TAG20180809T144939 comment=NONE
channel ORA_DISK_1: backup set complete, elapsed time: 00:00:01
channel ORA_DISK_1: starting full datafile backup set
channel ORA_DISK_1: specifying datafile(s) in backup set
input datafile file number=00008 name=/u01/app/oracle/oradata/PROD7/users04.dbf
channel ORA_DISK_1: starting piece 1 at 09-AUG-18
channel ORA_DISK_1: finished piece 1 at 09-AUG-18
piece handle=/u01/backup/users05ta4lg6_1_1 tag=TAG20180809T144939 comment=NONE
channel ORA_DISK_1: backup set complete, elapsed time: 00:00:01
channel ORA_DISK_1: starting full datafile backup set
channel ORA_DISK_1: specifying datafile(s) in backup set
input datafile file number=00004 name=/u01/app/oracle/oradata/PROD7/users01.dbf
channel ORA_DISK_1: starting piece 1 at 09-AUG-18
channel ORA_DISK_1: finished piece 1 at 09-AUG-18
piece handle=/u01/backup/users06ta4lg7_1_1 tag=TAG20180809T144939 comment=NONE
channel ORA_DISK_1: backup set complete, elapsed time: 00:00:01
Finished backup at 09-AUG-18
RMAN> backup filesperset 4 tablespace users format ‘/u01/backup/users%U’;
Starting backup at 09-AUG-18
using channel ORA_DISK_1
channel ORA_DISK_1: starting full datafile backup set
channel ORA_DISK_1: specifying datafile(s) in backup set
input datafile file number=00006 name=/u01/app/oracle/oradata/PROD7/users02.dbf
input datafile file number=00007 name=/u01/app/oracle/oradata/PROD7/users03.dbf
input datafile file number=00008 name=/u01/app/oracle/oradata/PROD7/users04.dbf
input datafile file number=00004 name=/u01/app/oracle/oradata/PROD7/users01.dbf
channel ORA_DISK_1: starting piece 1 at 09-AUG-18
channel ORA_DISK_1: finished piece 1 at 09-AUG-18
piece handle=/u01/backup/users07ta4ljp_1_1 tag=TAG20180809T145137 comment=NONE
channel ORA_DISK_1: backup set complete, elapsed time: 00:00:01
Finished backup at 09-AUG-18
RMAN> backup filesperset 5 tablespace users format ‘/u01/backup/users%U’;
Starting backup at 09-AUG-18
using channel ORA_DISK_1
channel ORA_DISK_1: starting full datafile backup set
channel ORA_DISK_1: specifying datafile(s) in backup set
input datafile file number=00006 name=/u01/app/oracle/oradata/PROD7/users02.dbf
input datafile file number=00007 name=/u01/app/oracle/oradata/PROD7/users03.dbf
input datafile file number=00008 name=/u01/app/oracle/oradata/PROD7/users04.dbf
input datafile file number=00004 name=/u01/app/oracle/oradata/PROD7/users01.dbf
channel ORA_DISK_1: starting piece 1 at 09-AUG-18
channel ORA_DISK_1: finished piece 1 at 09-AUG-18
piece handle=/u01/backup/users08ta4lk1_1_1 tag=TAG20180809T145145 comment=NONE
channel ORA_DISK_1: backup set complete, elapsed time: 00:00:01
Finished backup at 09-AUG-18
RMAN>