1.Oracle参考文档中关于RMAN备份压缩的描述
1）关于如何通过调整RMAN参数启用取消备份压缩功能
http://download.oracle.com/docs/cd/B19306_01/backup.102/b14192/setup004.htm#sthref229
使用关键字COMPRESSED启用RMAN压缩备份方法
RMAN> CONFIGURE DEVICE TYPE DISK BACKUP TYPE TO COMPRESSED BACKUPSET;
去掉COMPRESSED关键字取消RMAN压缩备份方法
RMAN> CONFIGURE DEVICE TYPE DISK BACKUP TYPE TO BACKUPSET;

2）关于如何直接使用RMAN命令启用备份压缩功能
http://download.oracle.com/docs/cd/B19306_01/backup.102/b14192/bkup002.htm#BRBSC138
两条压缩备份的RMAN语句，第一条语句实现对整个数据库及归档日志进行压缩备份，第二条命令实现对1、2和4号数据文件进行压缩备份。
BACKUP AS COMPRESSED BACKUPSET DATABASE PLUS ARCHIVELOG;
BACKUP AS COMPRESSED BACKUPSET DATAFILE 1,2,4;

2.测试压缩备份
1）测试脚本
（1）具有压缩功能的测试脚本
[oracle@secdb rmanbak]$ cat test_rman_compress.sh
date
rman target / <<EOF
backup as compressed backupset full database format '/u01/app/oracle/rmanbak/full_bk1_%u%p%s.rmn';
exit;
EOF
date

（2）不含压缩功能的测试脚本
[oracle@secdb rmanbak]$ cat test_rman_nocompress.sh
date
rman target / <<EOF
CONFIGURE DEVICE TYPE DISK BACKUP TYPE TO BACKUPSET;
backup full database format '/u01/app/oracle/rmanbak/full_bk1_%u%p%s.rmn';
exit;
EOF
date

3.执行RMAN测试脚本及结论分析
1）具有压缩功能的测试脚本执行输出结果
[oracle@secdb rmanbak]$ sh test_rman_compress.sh
Thu Jul 21 22:29:46 CST 2010

Recovery Manager: Release 10.2.0.1.0 - Production on Thu Jul 21 22:29:46 2010

Copyright (c) 1982, 2005, Oracle.  All rights reserved.

connected to target database: SECOOLER (DBID=4067316777)

RMAN>
Starting backup at 21-JUL-10
using target database control file instead of recovery catalog
allocated channel: ORA_DISK_1
channel ORA_DISK_1: sid=152 devtype=DISK
channel ORA_DISK_1: starting compressed full datafile backupset
channel ORA_DISK_1: specifying datafile(s) in backupset
input datafile fno=00001 name=/u01/app/oracle/oradata/secooler/system01.dbf
input datafile fno=00017 name=/u01/app/oracle/oradata/secooler/tbs_perf_01.dbf
input datafile fno=00003 name=/u01/app/oracle/oradata/secooler/sysaux01.dbf
input datafile fno=00005 name=/u01/app/oracle/oradata/secooler/example01.dbf
input datafile fno=00007 name=/u01/app/oracle/oradata/secooler/tbs_rman_catalog01.dbf
input datafile fno=00002 name=/u01/app/oracle/oradata/secooler/undotbs01.dbf
input datafile fno=00006 name=/u01/app/oracle/oradata/secooler/user_08.dbf
input datafile fno=00018 name=/u01/app/oracle/oradata/secooler/EXAMPLE1_01.dbf
input datafile fno=00019 name=/u01/app/oracle/oradata/secooler/OLTP_01.dbf
input datafile fno=00020 name=/u01/app/oracle/oradata/secooler/OLTP_02.dbf
input datafile fno=00008 name=/u01/app/oracle/oradata/secooler/tbs1.dbf
input datafile fno=00014 name=/u01/app/oracle/oradata/secooler/indx_01.dbf
input datafile fno=00016 name=/u01/app/oracle/oradata/secooler/tbs_assm_01.dbf
input datafile fno=00004 name=/u01/app/oracle/oradata/secooler/users01.dbf
channel ORA_DISK_1: starting piece 1 at 21-JUL-10
channel ORA_DISK_1: finished piece 1 at 21-JUL-10
piece handle=/u01/app/oracle/rmanbak/full_bk1_2nlje62e187.rmn tag=TAG20100721T222949 comment=NONE
channel ORA_DISK_1: backup set complete, elapsed time: 00:01:25
channel ORA_DISK_1: starting compressed full datafile backupset
channel ORA_DISK_1: specifying datafile(s) in backupset
input datafile fno=00009 name=/u01/app/oracle/oradata/secooler/DATA01_02.dbf
input datafile fno=00010 name=/u01/app/oracle/oradata/secooler/DATA01_03.dbf
input datafile fno=00011 name=/u01/app/oracle/oradata/secooler/DATA01_04.dbf
input datafile fno=00012 name=/u01/app/oracle/oradata/secooler/DATA01_05.dbf
input datafile fno=00013 name=/u01/app/oracle/oradata/secooler/DATA01_01.dbf
input datafile fno=00015 name=/u01/app/oracle/oradata/secooler/lob_data01.dbf
channel ORA_DISK_1: starting piece 1 at 21-JUL-10
channel ORA_DISK_1: finished piece 1 at 21-JUL-10
piece handle=/u01/app/oracle/rmanbak/full_bk1_2olje653188.rmn tag=TAG20100721T222949 comment=NONE
channel ORA_DISK_1: backup set complete, elapsed time: 00:00:07
Finished backup at 21-JUL-10

Starting Control File and SPFILE Autobackup at 21-JUL-10
piece handle=/u01/app/oracle/rmanbak/cf_c-4067316777-20100721-0e comment=NONE
Finished Control File and SPFILE Autobackup at 21-JUL-10

RMAN>

Recovery Manager complete.
Thu Jul 21 22:31:30 CST 2010

2）不含压缩功能的测试脚本执行输出结果
[oracle@secdb rmanbak]$ sh test_rman_nocompress.sh
Thu Jul 21 22:39:29 CST 2010

Recovery Manager: Release 10.2.0.1.0 - Production on Thu Jul 21 22:39:29 2010

Copyright (c) 1982, 2005, Oracle.  All rights reserved.

connected to target database: SECOOLER (DBID=4067316777)

RMAN>
using target database control file instead of recovery catalog
old RMAN configuration parameters:
CONFIGURE DEVICE TYPE DISK BACKUP TYPE TO BACKUPSET PARALLELISM 1;
new RMAN configuration parameters:
CONFIGURE DEVICE TYPE DISK BACKUP TYPE TO BACKUPSET PARALLELISM 1;
new RMAN configuration parameters are successfully stored

RMAN>
Starting backup at 21-JUL-10
allocated channel: ORA_DISK_1
channel ORA_DISK_1: sid=152 devtype=DISK
channel ORA_DISK_1: starting full datafile backupset
channel ORA_DISK_1: specifying datafile(s) in backupset
input datafile fno=00001 name=/u01/app/oracle/oradata/secooler/system01.dbf
input datafile fno=00017 name=/u01/app/oracle/oradata/secooler/tbs_perf_01.dbf
input datafile fno=00003 name=/u01/app/oracle/oradata/secooler/sysaux01.dbf
input datafile fno=00005 name=/u01/app/oracle/oradata/secooler/example01.dbf
input datafile fno=00007 name=/u01/app/oracle/oradata/secooler/tbs_rman_catalog01.dbf
input datafile fno=00002 name=/u01/app/oracle/oradata/secooler/undotbs01.dbf
input datafile fno=00006 name=/u01/app/oracle/oradata/secooler/user_08.dbf
input datafile fno=00018 name=/u01/app/oracle/oradata/secooler/EXAMPLE1_01.dbf
input datafile fno=00019 name=/u01/app/oracle/oradata/secooler/OLTP_01.dbf
input datafile fno=00020 name=/u01/app/oracle/oradata/secooler/OLTP_02.dbf
input datafile fno=00008 name=/u01/app/oracle/oradata/secooler/tbs1.dbf
input datafile fno=00014 name=/u01/app/oracle/oradata/secooler/indx_01.dbf
input datafile fno=00016 name=/u01/app/oracle/oradata/secooler/tbs_assm_01.dbf
input datafile fno=00004 name=/u01/app/oracle/oradata/secooler/users01.dbf
channel ORA_DISK_1: starting piece 1 at 21-JUL-10
channel ORA_DISK_1: finished piece 1 at 21-JUL-10
piece handle=/u01/app/oracle/rmanbak/full_bk1_2tlje6kl193.rmn tag=TAG20100721T223932 comment=NONE
channel ORA_DISK_1: backup set complete, elapsed time: 00:01:05
channel ORA_DISK_1: starting full datafile backupset
channel ORA_DISK_1: specifying datafile(s) in backupset
input datafile fno=00009 name=/u01/app/oracle/oradata/secooler/DATA01_02.dbf
input datafile fno=00010 name=/u01/app/oracle/oradata/secooler/DATA01_03.dbf
input datafile fno=00011 name=/u01/app/oracle/oradata/secooler/DATA01_04.dbf
input datafile fno=00012 name=/u01/app/oracle/oradata/secooler/DATA01_05.dbf
input datafile fno=00013 name=/u01/app/oracle/oradata/secooler/DATA01_01.dbf
input datafile fno=00015 name=/u01/app/oracle/oradata/secooler/lob_data01.dbf
channel ORA_DISK_1: starting piece 1 at 21-JUL-10
channel ORA_DISK_1: finished piece 1 at 21-JUL-10
piece handle=/u01/app/oracle/rmanbak/full_bk1_2ulje6mn194.rmn tag=TAG20100721T223932 comment=NONE
channel ORA_DISK_1: backup set complete, elapsed time: 00:00:15
Finished backup at 21-JUL-10

Starting Control File and SPFILE Autobackup at 21-JUL-10
piece handle=/u01/app/oracle/rmanbak/cf_c-4067316777-20100721-10 comment=NONE
Finished Control File and SPFILE Autobackup at 21-JUL-10

RMAN>

Recovery Manager complete.
Thu Jul 21 22:41:06 CST 2010

3）比较时间消耗
从elapsed time可以得出以下信息：
channel ORA_DISK_1: backup set complete, elapsed time: 00:01:25
channel ORA_DISK_1: backup set complete, elapsed time: 00:00:07

channel ORA_DISK_1: backup set complete, elapsed time: 00:01:05
channel ORA_DISK_1: backup set complete, elapsed time: 00:00:15

启用压缩耗时：  00:01:25 + 00:00:07 = 00:01:32
未启用压缩耗时：00:01:05 + 00:00:15 = 00:01:20

结论：启用压缩备份耗时略有增加（在数据量较大的环境下这个时间差会比较大）。

4）从top监控CPU使用率上进行比较
启用压缩备份期间CPU使用率监控结果
  PID USER      PR  NI  VIRT  RES  SHR S %CPU %MEM    TIME+  COMMAND
 4972 oracle    25   0  424m  52m  23m R 65.4  0.6   0:32.62 oracle

未启用压缩备份期间CPU使用率监控结果
  PID USER      PR  NI  VIRT  RES  SHR S %CPU %MEM    TIME+  COMMAND
 5902 oracle    25   0  423m  47m  25m R 33.1  0.6   0:09.17 oracle

结论：启用压缩CPU使用率在65.4%左右，未启用时的CPU使用率在33.1%左右。差别很明显，启用压缩将消耗更多的CPU资源。

5）比较生成的备份集大小
压缩备份集大小
[oracle@secdb rmanbak]$ du -sm /u01/app/oracle/rmanbak/full_bk1_2nlje62e187.rmn
168     /u01/app/oracle/rmanbak/full_bk1_2nlje62e187.rmn

未压缩备份集大小
[oracle@secdb rmanbak]$ du -sm /u01/app/oracle/rmanbak/full_bk1_2tlje6kl193.rmn
951     /u01/app/oracle/rmanbak/full_bk1_2tlje6kl193.rmn

结论：启用压缩功能，备份集大小是168M，而未启用压缩功能的备份集大小有951M大，是压缩备份集的5.7倍。这就是压缩备份带来的最大好处。

4.小结
使用RMAN备份压缩技术可以在很大程度上节省存储备份的空间，是典型的以时间和CPU换取空间的做法。如果系统有合理的备份窗口，不妨将这项技术引入到日常备份中去。

Good luck.

secooler
10.07.21

-- The End --

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