Recovery Manager Advanced Techniques - Oracle 10g/11g Data and Database Management Utilities - page 103

Improving data set compression with the ZLIB
algorithm (11g only)
One new feature in 11g is more efficient compression, using the ZLIB Algorithm.
In previous releases, RMAN used the BZIP2 algorithm for backupset compression;
however, there was a CPU cost penalty involved. 11g introduced a ZLIB
algorithm, which is less aggressive in terms of CPU usage. The user can decide
which compression algorithm to use by setting a value to the COMPRESSION
ALGORITHM RMAN configuration parameter.
The default value for the Compression algorithm in 11g is:
CONFIGURE COMPRESSION ALGORITHM 'ZLIB';
If you want to revert to the BZIP2 algorithm, just change the setting to reflect this:
CONFIGURE COMPRESSION ALGORITHM 'BZIP2';
If your system doesn't see the CPU compromised when performing a compressed
backup you can consider this technique. If you are performing a backup against a
tape device that also performs some sort of compression, don't mix both techniques,
use either that of the tape or that of RMAN. Mixing both won't lead to a satisfactory
result as you cannot compress an already compressed data set.