Apache generally logs all the web server access events in the access_log file, whereas errors and warnings go into the error_log file. The access_log file can later be analyzed to report server usage statistics, such as the number of requests made in different time spans, who issued these requests, and much more. The error_log file is used to monitor the server for errors and warnings and to prompt actions based on those reports. Some systems do additional logging, such as storing the referrers of incoming requests to find out how users have learned about the site.
The simplest logging technique is to dump the logs into a file opened for appending. With Apache, this is as simple as specifying the logging format and the file to which to log. For example, to log all accesses, use the default directive supplied in httpd.conf:
LogFormat "%h %l %u %t \"%r\" %>s %b" common CustomLog /home/httpd/httpd_perl/logs/access_log common
This setting will log all server accesses to a file named /home/httpd/httpd_perl/logs/access_log using the format specified by the LogFormat directive—in this case, common. Please refer to the Apache documentation for a complete explanation of the various tokens that you can use when specifying log formats. If you're tempted to change the format of the log file, bear in mind that some log analysis tools may expect that only the default or one of a small subset of logging formats is used.
The only risk with log files is their size. It is important to keep log files trimmed. If they are needed for later analysis, they should be rotated and the rotation files should be moved somewhere else so they do not consume disk space. You can usually compress them for storage offline.
The most important thing is to monitor log files for possible sudden explosive growth rates. For example, if a developer makes a mistake in his code running on the mod_perl server and the child processes executing the code start to log thousands of error messages a second, all disk space can quickly be consumed, and the server will cease to function.