这个压缩比例可以和操作系统“gzip -9”相媲美，某些特例下有可能比gzip还要高效。体验之，供参考。
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1.Oracle 11g中expdp帮助页中关于COMPRESSION参数的描述
secooler@secDB /home/oracle$ expdp -help
……
COMPRESSION
Reduce the size of a dump file.
Valid keyword values are: ALL, DATA_ONLY, [METADATA_ONLY] and NONE.
……
可见，11g中的COMPRESSION参数提供四个选项，分别是ALL、DATA_ONLY、METADATA_ONLY和NONE，非常的丰富，稍后我们将使用ALL参数进行操作。
2.Oracle 10g中关于COMPRESSION参数的描述如下
Keyword Description (Default)
---------------------------------------------------------------------
……
COMPRESSION Reduce size of dumpfile contents where valid
keyword values are: (METADATA_ONLY) and NONE.
……
10g中的COMPRESSION参数只提供METADATA_ONLY和NONE两个选项，基本上没有提供压缩功能。
3.使用11g EXPDP工具的四个压缩参数分别生成四个dump文件
1）使用ALL参数
secooler@secDB /expdp$ expdp sec/sec dumpfile=sec_ALL.dmp logfile=sec_ALL.log tables=t directory=dump_dir compression=ALL
Export: Release 11.2.0.1.0 - Production on Sat Mar 20 22:48:52 2010
Copyright (c) 1982, 2009, Oracle and/or its affiliates. All rights reserved.
Connected to: Oracle Database 11g Enterprise Edition Release 11.2.0.1.0 - 64bit Production
With the Partitioning, Oracle Label Security, OLAP, Data Mining,
Oracle Database Vault and Real Application Testing options
Starting "SEC"."SYS_EXPORT_TABLE_01": sec/******** dumpfile=sec_ALL.dmp logfile=sec_ALL.log tables=t directory=dump_dir compression=ALL
Estimate in progress using BLOCKS method...
Processing object type TABLE_EXPORT/TABLE/TABLE_DATA
Total estimation using BLOCKS method: 136 MB
Processing object type TABLE_EXPORT/TABLE/TABLE
. . exported "SEC"."T" 14.26 MB 1155520 rows
Master table "SEC"."SYS_EXPORT_TABLE_01" successfully loaded/unloaded
******************************************************************************
Dump file set for SEC.SYS_EXPORT_TABLE_01 is:
/expdp/sec_ALL.dmp
Job "SEC"."SYS_EXPORT_TABLE_01" successfully completed at 22:49:08
2）使用DATA_ONLY参数
secooler@secDB /expdp$ expdp sec/sec dumpfile=sec_DATA_ONLY.dmp logfile=sec_DATA_ONLY.log tables=t directory=dump_dir compression=DATA_ONLY
Export: Release 11.2.0.1.0 - Production on Sat Mar 20 22:49:32 2010
Copyright (c) 1982, 2009, Oracle and/or its affiliates. All rights reserved.
Connected to: Oracle Database 11g Enterprise Edition Release 11.2.0.1.0 - 64bit Production
With the Partitioning, Oracle Label Security, OLAP, Data Mining,
Oracle Database Vault and Real Application Testing options
Starting "SEC"."SYS_EXPORT_TABLE_01": sec/******** dumpfile=sec_DATA_ONLY.dmp logfile=sec_DATA_ONLY.log tables=t directory=dump_dir compression=DATA_ONLY
Estimate in progress using BLOCKS method...
Processing object type TABLE_EXPORT/TABLE/TABLE_DATA
Total estimation using BLOCKS method: 136 MB
Processing object type TABLE_EXPORT/TABLE/TABLE
. . exported "SEC"."T" 14.26 MB 1155520 rows
Master table "SEC"."SYS_EXPORT_TABLE_01" successfully loaded/unloaded
******************************************************************************
Dump file set for SEC.SYS_EXPORT_TABLE_01 is:
/expdp/sec_DATA_ONLY.dmp
Job "SEC"."SYS_EXPORT_TABLE_01" successfully completed at 22:49:48
3）使用METADATA_ONLY参数
secooler@secDB /expdp$ expdp sec/sec dumpfile=sec_METADATA_ONLY.dmp logfile=sec_METADATA_ONLY.log tables=t directory=dump_dir compression=METADATA_ONLY
Export: Release 11.2.0.1.0 - Production on Sat Mar 20 22:50:16 2010
Copyright (c) 1982, 2009, Oracle and/or its affiliates. All rights reserved.
Connected to: Oracle Database 11g Enterprise Edition Release 11.2.0.1.0 - 64bit Production
With the Partitioning, Oracle Label Security, OLAP, Data Mining,
Oracle Database Vault and Real Application Testing options
Starting "SEC"."SYS_EXPORT_TABLE_01": sec/******** dumpfile=sec_METADATA_ONLY.dmp logfile=sec_METADATA_ONLY.log tables=t directory=dump_dir compression=METADATA_ONLY
Estimate in progress using BLOCKS method...
Processing object type TABLE_EXPORT/TABLE/TABLE_DATA
Total estimation using BLOCKS method: 136 MB
Processing object type TABLE_EXPORT/TABLE/TABLE
. . exported "SEC"."T" 111.7 MB 1155520 rows
Master table "SEC"."SYS_EXPORT_TABLE_01" successfully loaded/unloaded
******************************************************************************
Dump file set for SEC.SYS_EXPORT_TABLE_01 is:
/expdp/sec_METADATA_ONLY.dmp
Job "SEC"."SYS_EXPORT_TABLE_01" successfully completed at 22:50:27
4）使用NONE参数
secooler@secDB /expdp$ expdp sec/sec dumpfile=sec_NONE.dmp logfile=sec_NONE.log tables=t directory=dump_dir compression=NONE
Export: Release 11.2.0.1.0 - Production on Sat Mar 20 22:50:43 2010
Copyright (c) 1982, 2009, Oracle and/or its affiliates. All rights reserved.
Connected to: Oracle Database 11g Enterprise Edition Release 11.2.0.1.0 - 64bit Production
With the Partitioning, Oracle Label Security, OLAP, Data Mining,
Oracle Database Vault and Real Application Testing options
Starting "SEC"."SYS_EXPORT_TABLE_01": sec/******** dumpfile=sec_NONE.dmp logfile=sec_NONE.log tables=t directory=dump_dir compression=NONE
Estimate in progress using BLOCKS method...
Processing object type TABLE_EXPORT/TABLE/TABLE_DATA
Total estimation using BLOCKS method: 136 MB
Processing object type TABLE_EXPORT/TABLE/TABLE
. . exported "SEC"."T" 111.7 MB 1155520 rows
Master table "SEC"."SYS_EXPORT_TABLE_01" successfully loaded/unloaded
******************************************************************************
Dump file set for SEC.SYS_EXPORT_TABLE_01 is:
/expdp/sec_NONE.dmp
Job "SEC"."SYS_EXPORT_TABLE_01" successfully completed at 22:50:53
4.比较生成的四个dump文件大小
secooler@secDB /expdp$ du -sm sec*.dmp
15 sec_ALL.dmp
15 sec_DATA_ONLY.dmp
112 sec_METADATA_ONLY.dmp
112 sec_NONE.dmp
secooler@secDB /expdp$ du -sb sec*.dmp
14987264 sec_ALL.dmp
14987264 sec_DATA_ONLY.dmp
117223424 sec_METADATA_ONLY.dmp
117223424 sec_NONE.dmp
实验表明，使用ALL参数和DATA_ONLY参数生成的备份文件基本一样大；使用METADATA_ONLY参数与NONE参数效果一样。
使用压缩与不进行压缩的比率是：15/112约等于1/7！
这才是真正的压缩。
5.使用“gzip -9”对未压缩的备份文件进行压缩，做一下比较
secooler@secDB /expdp$ gzip -9 sec_NONE.dmp
secooler@secDB /expdp$ du -sm sec*dmp*
15 sec_ALL.dmp
15 sec_DATA_ONLY.dmp
112 sec_METADATA_ONLY.dmp
15 sec_NONE.dmp.gz
secooler@secDB /expdp$ du -sb sec*dmp*
14987264 sec_ALL.dmp
14987264 sec_DATA_ONLY.dmp
117223424 sec_METADATA_ONLY.dmp
14717055 sec_NONE.dmp.gz
14987264与14717055的区别，相差甚微，基本相同，“gzip -9”稍胜一点点（也许在您的具体环境下“gzip -9”会逊色一点点）。
6.小结
Oracle 11g的EXPDP工具提供了真正意义上的“备份压缩”，这个技术在备份空间不足的情况下非常实用。
Oracle在改进和优化中不断的前进，慢慢体会吧。