053-53

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.

ZLIB (压缩率低消耗CPU资源少速度快)运行的比 BZIP2(压缩率高消耗CPU资源多速度慢) 更快，但是产生更大的文件。 ZLIB 需要高级压缩选项。
可以使用 CONFIGURE COMPRESSION ALGORITHM ‘ZLIB’;来配置。
可以通过查询 select * from V$RMAN_COMPRESSION_ALGORITHM 来确定当前的算法（默认是 BASIS default）
可以使用以下级别或压缩比率：
• LOW：此级别速度最快。与 MEDIUM 相比，它提供较少的压缩，但 CPU 使用率最低。（与 LZO 压缩对应。）
• MEDIUM：此级别很好兼顾了 CPU 使用率和压缩比率。（与 ZLIB 压缩对应。）
• HIGH：此级别提供最佳压缩比率，但 CPU 消耗最大。（与 GZIP 压缩对应。）
• BASIC：与 BZIP2（10g 类型压缩）对应。 

 BASIC – default compression algorithm
 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 impact on backup throughput and suited for environments where CPU resources are the limiting factor.