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The following page has been changed by slive:
http://wiki.apache.org/httpd/Logs/Response_Size

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In the default log formats, the size of the response is logged using the 
{{{%b}}} directive in the log format string. This logs the size of the object 
requested by the client, but not necessarily the number of bytes actually 
delivered to the client. In other words, the log will contain exactly what is 
written in the {{{Content-Length}}} HTTP response header (for responses that 
use this header).

If you wish to instead log the number of bytes actually sent to the client, you 
should use the {{{%O}}} log format directive provided by {{{mod_logio}}}.

The difference between {{{%b}}} and {{{%O}}} will be especially large in cases 
where download accelerators are used. Download accelerators often make many 
simultaneous overlapping requests for the same object, using the {{{Range}}} 
header to request that the download start from someplace other than the 
beginning. When they receive what they want, they simply drop all the 
connections. The result is that the amount of content that Apache httpd 
believes it will send based on the requests ({{{%b}}}) winds up much larger 
than the amount of content actually sent ({{{%O}}}). This situation can be 
identified by looking for {{{206 Partial Content}}} status codes in the logs.   
 

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