This determines the level of compression used when doing a backup. The 
following options are available:

low

Uses either no compression (for partimage and dd) or the lzo 
compression algorithm (for fsarchiver). The advantage is that it's 
very fast on the client side. The disadvantage is that the size of 
the backup will be large; in most cases as large as the original 
data. Using this setting really only makes sense if you have vast 
amounts of storage space for your backups and a very fast network, 
or if you know that your data is pretty much uncompressible, such as 
a compressed database. Note that even though low compression is fast 
on the client side, you may still run into speed limitations of the 
storage subsystem on the server side, or of the network.

normal

Uses the gzip compression algorithm to give the best compromise 
between speed and backup size. This is the default compression level.

high

Uses the bzip2 compression algorithm (for partimage and dd) or the 
lzma compression algorithm (for fsarchiver). This results in a very 
good compression ratio requiring less space to store the backup. 
However, this comes at the price of high CPU and memory usage on the 
client side. A backup done with high compression will probably take 
a long, long time to accomplish. Additionally, if there is 
insufficient memory available on the client, the compression may 
fail altogether, resulting in the backup being stored uncompressed. 
If you want to use high compression, testing is advised before 
deciding to use it for real.