When logging with log4net to a file (using the FileAppender), the FileAppender is holding an exclusive lock on the file. This doesn’t cause any problems, not even when the application is running with multiple threads, because log4net should be thread-safe.

This does change however when working with multiple processes, that all share a common log4net configuration and thus all will utilize the same FileAppender. In this case there should not be an exclusive lock by any process. Fortunately log4net has an appropriate configuration-setting.

<lockingModel type="log4net.Appender.FileAppender+MinimalLock" />

But acquiring and release locks is quite costly and thus will slow down the overall performance of the application.

An alternative would be to create an individual log per processes. Luckily log4net supports the expansion of variables to generate log-filenames, so we can add the process-id to the filename-pattern.

<appender name="LogFileAppender" type="log4net.Appender.RollingFileAppender,log4net">
    <file type="log4net.Util.PatternString" value="Log[%processid]" />
[...]
</appender>