052试题 53/688 - Compression Option
题目：
53.You want to use RMAN to create compressed backups.
Which statement is true about the compression algorithms that RMAN can use?
A. The BZIP2 compression algorithm consumes more CPU resources than the ZLIB compression algorithm.
B. The ZLIB compression algorithm consumes more CPU resources than the BZIP2 compression algorithm.
C. The ZLIB compression algorithm provides maximum compression and produces smaller backups than the BZIP2 compression algorithm.
D. Only the BZIP2 compression algorithm can be used to make compressed backups to disk.
参考答案 A
解析
题目意思是，当打算在RMAN中使用压缩算法的时候，那个说法是正确的。
压缩级别对应的压缩算法：
LOW - 对应LZO压缩
MEDIUM -- 对应ZLIB压缩
HIGH --对应GZIP压缩
BASIC --对应BZIP2压缩
压缩率，从low到high，依次消耗的CPU是增加的。
bzip2压缩算法，压缩率是basic，比medium(zlib) 消耗更多的CPU。所以选择A 。
参考文档：
https://docs.oracle.com/cd/E11882_01/backup.112/e10642/rcmconfa.htm#BRADV89466
Basic Compression Option
You can configure the basic compression algorithm, which does not require the Advanced Compression Option, with the following syntax:
Example 6-2 Configuring Basic Compression for Backup
CONFIGURE COMPRESSION ALGORITHM 'BASIC';
Advanced Compression Option
If you have enabled the Oracle Database 11g Release 2 Advanced Compression Option, you can choose from the following compression levels:
| Compression Level | Performance Benefits and Trade-Offs |
|---|---|
HIGH |
Best suited for backups over slower networks where the limiting factor is network speed. |
MEDIUM |
Recommended for most environments. Good combination of compression ratios and speed. |
LOW |
Least effect on backup throughput. |
The compression ratio generally increases from low to high, with a trade-off of potentially consuming more CPU resources.
Because the performance of the various compression levels depends on the nature of the data in the database, network configuration, system resources and the type of computer system and its capabilities, Oracle cannot document universally applicable performance statistics. Which level is best for your environment depends on how balanced your system is regarding bandwidth into the CPU and the actual speed of the CPU. It is highly recommended that you run tests with the different compression levels on the data in your environment. Choosing a compression level based on your environment, network traffic characteristics (workload), and data set is the only way to ensure that the backup set compression level can satisfy your organization's performance requirements and applicable service level agreements.
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-- 2019-09-12 add
题目：
688. Which two statements are true about the compressed backups in RMAN? (Choose two.)
A.Compressed backups can only be taken on the tape drives.
B.The binary compression creates some performance overhead during backup operation.
C.The ZLIB compression algorithm can be used only if the COMPATIBLE initialization parameter is set to 11.1.0.
D.The media manager compression for the tape drive should be enabled for taking compressed backups on the tape.
参考答案 BD
解析
rman的压缩备份
rman的压缩备份是可以在磁盘上的。A错误。
二进制压缩在备份操作期间会产生一些性能开销。主要是CPU开销。B正确。
ZIB压缩算法的兼容性是11.0 C错误。
应该启用磁带驱动器的媒体管理器压缩，以便对磁带进行压缩备份 D正确。
参考文档，参考上面的题目的文档。
SYS@test> select ALGORITHM_NAME,ALGORITHM_COMPATIBILITY,ALGORITHM_DESCRIPTION from v$rman_compression_algorithm;
ALGORITHM_NAME ALGORITHM_COMPATIBIL ALGORITHM_DESCRIPTION
---------------------------------------------------------------- -------------------- ------------------------------------------------------------
BZIP2 9.2.0.0.0 good compression ratio
BASIC 9.2.0.0.0 good compression ratio
LOW 11.2.0.0.0 maximum possible compression speed
ZLIB 11.0.0.0.0 balance between speed and compression ratio
MEDIUM 11.0.0.0.0 balance between speed and compression ratio
HIGH 11.2.0.0.0 maximum possible compression ratio
6 rows selected.
SYS@test>
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