Mod_gzip is another extension to the common log format. It is used by the mod_gzip Apache™ extension which can be used to compress the result of requests before sending them to the client.
mod_gzip is a module developed by RemoteCommunications, Inc. Sourcecode is freely available from http://www.RemoteCommunications.com/apache/mod_gzip/mod_gzip. More informations can be found in their FAQ.
mod_gzip can log information about the compression of pages. To enable this, one can configure Apache™ to log using the 'gzip' format which can be defined as follows:
LogFormat "%h %l %u %t \"%r\" %>s %b %{mod_gzip_result}n \ %{mod_gzip_compression_ratio}n" gzip
This adds two fields at the end of each common log line:
gzip_result compression_ratio
The gzip result code. Usually OK.
The ratio by which the content was compressed. A number from 0 to 100.