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There are two ways to modify the logging levels, as described in Working with Confluence Logs.
<Confluence-Install>\confluence\WEB-INF\classes\log4j.properties file. The information on this page does not apply to Confluence Cloud.
The standard Confluence log level WARN is a way for Confluence to communicate with the server administrator. Logging at WARN level and higher should be reserved for situations that require some kind of attention from the server administrator, and for which corrective action is possible.
Reference : log4j manual