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4.3.8 创建多段备份
创建多个段是一件易如反掌的事情，但是必须使用每个BACKUP命令指定段大小。另外，可以按段运行RMAN VALIDATE命令。新数据字典视图V$和RC_帮助你确定哪些备份是多段备份，以及多段备份中的每个段包含多少个数据块。
1．指定段大小
要创建多段备份，可以为BACKUP命令添加SECTION SIZE参数。可按KB、MB或GB指定段大小。以下是指定多段备份的常用语法：
BACKUP <backup options> SECTION SIZE <size> [K|M|G]
在本例的HR数据库中，USERS表空间约为25MB，并且在备份该表空间时希望使用10MB的段大小：
RMAN> backup tablespace users
2> section size 100m;
Starting backup at 25-FEB-14
using channel ORA_DISK_1
using channel ORA_DISK_2
using channel ORA_DISK_3
using channel ORA_DISK_4
channel ORA_DISK_1: starting compressed full datafile backup set
channel ORA_DISK_1: specifying datafile(s) in backup set
input datafile file number=00005
name=+DATA/RPT12C/DATAFILE/users.269.826931347
backing up blocks 1 through 12800
channel ORA_DISK_1: starting piece 1 at 25-FEB-14
channel ORA_DISK_2: starting compressed full datafile backup set
channel ORA_DISK_2: specifying datafile(s) in backup set
input datafile file number=00006
name=+DATA/RPT12C/DATAFILE/users.259.826650843
channel ORA_DISK_2: starting piece 1 at 25-FEB-14
channel ORA_DISK_3: starting compressed full datafile backup set
channel ORA_DISK_3: specifying datafile(s) in backup set
input datafile file number=00005
name=+DATA/RPT12C/DATAFILE/users.269.826931347
backing up blocks 12801 through 25600
channel ORA_DISK_3: starting piece 2 at 25-FEB-14
channel ORA_DISK_4: starting compressed full datafile backup set
channel ORA_DISK_4: specifying datafile(s) in backup set
input datafile file number=00005
name=+DATA/RPT12C/DATAFILE/users.269.826931347
backing up blocks 25601 through 32000
channel ORA_DISK_4: starting piece 3 at 25-FEB-14
channel ORA_DISK_1: finished piece 1 at 25-FEB-14
piece handle=+RECOV/RPT12C/BACKUPSET/2014_02_25/
nnndf0_tag20140225t101644_0.278.840449805 tag=TAG20140225T101644
comment=NONE
channel ORA_DISK_1: backup set complete, elapsed time: 00:00:01
channel ORA_DISK_2: finished piece 1 at 25-FEB-14
piece handle=+RECOV/RPT12C/BACKUPSET/2014_02_25/
nnndf0_tag20140225t101644_0.277.840449805 tag=TAG20140225T101644
comment=NONE
channel ORA_DISK_2: backup set complete, elapsed time: 00:00:01
channel ORA_DISK_3: finished piece 2 at 25-FEB-14
piece handle=+RECOV/RPT12C/BACKUPSET/2014_02_25/
nnndf0_tag20140225t101644_0.276.840449807 tag=TAG20140225T101644
comment=NONE
channel ORA_DISK_3: backup set complete, elapsed time: 00:00:01
channel ORA_DISK_4: finished piece 3 at 25-FEB-14
piece handle=+RECOV/RPT12C/BACKUPSET/2014_02_25/
nnndf0_tag20140225t101644_0.275.840449807 tag=TAG20140225T101644
comment=NONE
channel ORA_DISK_4: backup set complete, elapsed time: 00:00:01
Finished backup at 25-FEB-14
RMAN>
此备份创建3个备份段：前两个段的大小都是100MB，第三个段的大小约为50MB，是数据文件剩余的大小。
提示：
在少量磁盘上备份大型文件时，不要为多段备份使用大的并发值。多个RMAN线程访问同一个磁盘时，由于产生了I/O争用，将会使高并发值带来的时间节省优势荡然无存。
2．使用段大小验证备份
还可以将SECTION SIZE参数与VALIDATE命令结合使用。数据文件块验证的并发操作的优势与使用BACKUP命令的优势是一样的：将使用少得多的时间确保数据文件的块是可读的，而且“校验和”有效。此例验证上一节中备份的数据文件：
RMAN> validate tablespace users
2> section size 100m;
Starting validate at 25-FEB-14
using channel ORA_DISK_1
using channel ORA_DISK_2
using channel ORA_DISK_3
using channel ORA_DISK_4
channel ORA_DISK_1: starting validation of datafile
channel ORA_DISK_1: specifying datafile(s) for validation
input datafile file number=00006
name=+DATA/RPT12C/DATAFILE/users.259.826650843
channel ORA_DISK_2: starting validation of datafile
channel ORA_DISK_2: specifying datafile(s) for validation
input datafile file number=00005
name=+DATA/RPT12C/DATAFILE/users.269.826931347
validating blocks 1 through 12800
channel ORA_DISK_3: starting validation of datafile
channel ORA_DISK_3: specifying datafile(s) for validation
input datafile file number=00005
name=+DATA/RPT12C/DATAFILE/users.269.826931347
validating blocks 12801 through 25600
channel ORA_DISK_4: starting validation of datafile
channel ORA_DISK_4: specifying datafile(s) for validation
input datafile file number=00005
name=+DATA/RPT12C/DATAFILE/users.269.826931347
validating blocks 25601 through 32000
channel ORA_DISK_1: validation complete, elapsed time: 00:00:02
List of Datafiles
=================
File Status Marked Corrupt Empty Blocks Blocks Examined High SCN
---- ------ -------------- --------------------------- ----------
6 OK 0 16 257 15654354
File Name: +DATA/RPT12C/DATAFILE/users.259.826650843
Block Type Blocks Failing Blocks Processed
---------- -------------- ----------------
Data 0 30
Index 0 5
Other 0 205
channel ORA_DISK_2: validation complete, elapsed time: 00:00:02
channel ORA_DISK_3: validation complete, elapsed time: 00:00:03
channel ORA_DISK_4: validation complete, elapsed time: 00:00:03
List of Datafiles
=================
File Status Marked Corrupt Empty Blocks Blocks Examined High SCN
---- ------ -------------- ------------ --------------- ----------
5 OK 0 31873 31998 1991172
File Name: +DATA/RPT12C/DATAFILE/users.269.826931347
Block Type Blocks Failing Blocks Processed
---------- -------------- ----------------
Data 0 0
Index 0 0
Other 0 125
Finished validate at 25-FEB-14
RMAN>
3．多段数据字典视图
V$BACKUP_SET和RC_BACKUP_SET视图具有MULTI_SECTION列，此列指示备份是否是多段备份。同样，V$BACKUP_DATAFILE和RC_BACKUP_DATAFILE视图具有SECTION_SIZE列，此列指示多段备份中每个段的块数。注意，V$视图仅在目标数据库上存在，而RC_视图仅在恢复目录数据库上存在。