Oracle10g允许你和BACKUP命令伴随使用AS COMPRESSED BACKUPSET字句，对备份集进行压缩。
这个压缩备份在恢复时无需解压缩，这一特性在以下条件下可能大为有益:
1.你的备份存储空间有限
2.通过网络备份而带宽有限
3.直接备份至磁带、CD，DVD介质而压缩不可用
你可以选择在备份命令里显示指定压缩选项:
BACKUP AS COMPRESSED BACKUPSET DATABASE PLUS ARCHIVELOG;
--仅对1,5文件压缩备份
BACKUP AS COMPRESSED BACKUPSET DATAFILE 1,5;
也可以配置RMAN指定压缩选项:
CONFIGURE DEVICE TYPE DISK PARALLELISM 1 BACKUP TYPE TO COMPRESSED BACKUPSET;
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下边来做个测试来计较下他们吧：
非压缩备份
RMAN> backup database;
Starting backup at 28-JUL-12
using channel ORA_DISK_1
channel ORA_DISK_1: starting full datafile backupset
channel ORA_DISK_1: specifying datafile(s) in backupset
input datafile fno=00001 name=/oracle/oradata/orcl/system01.dbf
input datafile fno=00003 name=/oracle/oradata/orcl/sysaux01.dbf
input datafile fno=00005 name=/oracle/oradata/orcl/jietbs01.dbf
input datafile fno=00002 name=/oracle/oradata/orcl/undotbs01.dbf
input datafile fno=00004 name=/oracle/oradata/orcl/users01.dbf
channel ORA_DISK_1: starting piece 1 at 28-JUL-12
channel ORA_DISK_1: finished piece 1 at 28-JUL-12
piece handle=/home/oracle/app/product/10.2.0/db_1/flash_recovery_area/ORCL/backupset/2012_07_28/o1_mf_nnndf_TAG20120728T011430_815m3q6m_.bkp tag=TAG20120728T011430 comment=NONE
channel ORA_DISK_1: backup set complete, elapsed time: 00:01:06
channel ORA_DISK_1: starting full datafile backupset
channel ORA_DISK_1: specifying datafile(s) in backupset
including current control file in backupset
including current SPFILE in backupset
channel ORA_DISK_1: starting piece 1 at 28-JUL-12
channel ORA_DISK_1: finished piece 1 at 28-JUL-12
piece handle=/home/oracle/app/product/10.2.0/db_1/flash_recovery_area/ORCL/backupset/2012_07_28/o1_mf_ncsnf_TAG20120728T011430_815m5tgv_.bkp tag=TAG20120728T011430 comment=NONE
channel ORA_DISK_1: backup set complete, elapsed time: 00:00:03
Finished backup at 28-JUL-12
消耗时间:
00:01:06+00:00:03=1分08秒=68s
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压缩备份
RMAN> backup as compressed backupset database;
Starting backup at 28-JUL-12
using channel ORA_DISK_1
channel ORA_DISK_1: starting compressed full datafile backupset
channel ORA_DISK_1: specifying datafile(s) in backupset
input datafile fno=00001 name=/oracle/oradata/orcl/system01.dbf
input datafile fno=00003 name=/oracle/oradata/orcl/sysaux01.dbf
input datafile fno=00005 name=/oracle/oradata/orcl/jietbs01.dbf
input datafile fno=00002 name=/oracle/oradata/orcl/undotbs01.dbf
input datafile fno=00004 name=/oracle/oradata/orcl/users01.dbf
channel ORA_DISK_1: starting piece 1 at 28-JUL-12
channel ORA_DISK_1: finished piece 1 at 28-JUL-12
piece handle=/home/oracle/app/product/10.2.0/db_1/flash_recovery_area/ORCL/backupset/2012_07_28/o1_mf_nnndf_TAG20120728T012227_815mln9b_.bkp tag=TAG20120728T012227 comment=NONE
channel ORA_DISK_1: backup set complete, elapsed time: 00:01:15
channel ORA_DISK_1: starting compressed full datafile backupset
channel ORA_DISK_1: specifying datafile(s) in backupset
including current control file in backupset
including current SPFILE in backupset
channel ORA_DISK_1: starting piece 1 at 28-JUL-12
channel ORA_DISK_1: finished piece 1 at 28-JUL-12
piece handle=/home/oracle/app/product/10.2.0/db_1/flash_recovery_area/ORCL/backupset/2012_07_28/o1_mf_ncsnf_TAG20120728T012227_815mo1kh_.bkp tag=TAG20120728T012227 comment=NONE
channel ORA_DISK_1: backup set complete, elapsed time: 00:00:03
Finished backup at 28-JUL-12
消耗时间:00:01:15+
00:00:03
=1分18秒=78s
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非压缩备份 CPU(18.1%)
PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND
5483 oracle 18 0 414m 72m 40m D 18.1 10.5 0:54.36 oracle
压缩备份 CPU(84.0%)
PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND
5483 oracle 18 0 430m 88m 48m R 84.0 27.8 0:47.09 oracle
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非压缩备份的大小(107724800+1097728=108822528)
-rw-r----- 1 oracle oinstall 107724800 Jul 28 01:34 o1_mf_nnndf_TAG20120728T013252_815n64k8_.bkp
-rw-r----- 1 oracle oinstall 1097728 Jul 28 01:34 o1_mf_ncsnf_TAG20120728T013252_815n8llv_.bkp
压缩备份的大小(599318528+7143424=606461952)
-rw-r----- 1 oracle oinstall 599318528 Jul 28 01:28 o1_mf_nnndf_TAG20120728T012730_815mw2yz_.bkp
-rw-r----- 1 oracle oinstall 7143424 Jul 28 01:28 o1_mf_ncsnf_TAG20120728T012730_815my5v5_.bkp
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下边我们来比较下他们之前的差别:
压缩时间(非压缩:压缩)
68s:78s 大约 4：5 (1:1.14)
压缩占磁盘的空间(非压缩:压缩) 103.78125:578.367188 大约 1：5
压缩CPU使用(非压缩:压缩) 18.1%:84.0% 大约 (1:4.67)
压缩占空间少，相对来说比较消耗CPU和时间，呵呵。
