In prudent mode, FileAppender will safely write to the specified file, even in the presence of other FileAppender instances running in different JVMs, potentially running on different hosts. The default value for prudent mode is false.Prudent mode can be used in conjunction with RollingFileAppender although some

restrictions apply.Prudent mode implies that append property is automatically set to true.Prudent more relies on exclusive file locks. Experiments show that file locks approximately triple (x3) the cost of writing a logging event. On an “average” PC writing to a file located on a local hard disk, when prudent mode is off, it takes about 10 microseconds to write a single logging event. When prudent mode is on, it takes approximately 30 microseconds to output a single logging event. This translates to logging throughput of 100’000 events per second when prudent mode is off and approximately 33’000 events per second in prudent mode.Prudent mode effectively serializes I/O operations between all JVMs writing to the same file. Thus, as the number of JVMs competing to access a file increases so will the delay incurred by each I/O operation. As long as the total number of I/O operations is in the order of 20 log requests per second, the impact on performance should be negligible. Applications generating 100 or more I/O operations per second can see an impact on performance and should avoid using prudent mode.NETWORKED FILE LOCKS When the log file is located on a networked file system, the cost of prudent mode is even greater. Just as importantly, file locks over a networked file system can be sometimes strongly biased such that the process currently owning the lock immediately re-obtains the lock upon its release. Thus, while one process hogs the lock for the log file, other processes starve waiting for the lock to the point of appearing deadlocked.The impact of prudent mode is highly dependent on network speed as well as the OS implementation details. We provide an very small application called

FileLockSimulator which can help you simulate the behavior of prudent mode in your environment.