NLog – Archive by file size

To archive by file size when you’re using NLog, you can configure nlog.config like this:

<?xml version="1.0" encoding="utf-8" ?>
<nlog xmlns="http://www.nlog-project.org/schemas/NLog.xsd"
      xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
      xsi:schemaLocation="http://www.nlog-project.org/schemas/NLog.xsd NLog.xsd"
      autoReload="true">


  <variable name ="logFile" value="C:/logs/servicelog-${shortdate}" />

  <targets>
    <target xsi:type="File"
            name="mainLog"
            fileName="${logFile}.log"
            layout="${longdate} level=${level} message=${message}"
            keepFileOpen ="false"
            concurrentWrites ="true"
            archiveNumbering="DateAndSequence"
            archiveAboveSize="1000000"
            maxArchiveFiles="10"/>
  </targets>

  <rules>
    <logger name="*" minlevel="Trace" writeTo="mainLog" />
  </rules>

</nlog>
Code language: HTML, XML (xml)

You specify archiveAboveSize in bytes. The above configuration is specifying ~1 MB.

When your log file hits the specified size, NLog will “archive” the file, which really just means it will rename the log file and start a new log file. It will rename the log file based on the setting archiveNumbering. It’s a good idea to set maxArchiveFiles as well, so you don’t run out of disk space.

Example of archiving by log file size

I’m logging to a log file with the short date in the name. The log file is called servicelog-2021-02-22.log.

Once servicelog-2021-02-22.log hits ~1 MB, NLog will rename servicelog-2021-02-22.log to servicelog-2021-02-22.20210222.0.log (because archiveNumbering=DateAndSequence) and then create a new file called servicelog-2021-02-22.log, like this:

servicelog-2021-02-22.log
servicelog-2021-02-22.20210222.0.log
Code language: plaintext (plaintext)

Let’s say I am logging all day and have 10 archived log files. Because maxArchiveFiles=10, when NLog has to archive again, it will delete the oldest archived file (servicelog-2021-02-22.20210222.0.log) and create the new archive log file with the next sequence number – servicelog-2021-02-22.20210222.10.log.

I’ll end up with the following log files:

servicelog-2021-02-22.log
servicelog-2021-02-22.20210222.10.log
servicelog-2021-02-22.20210222.9.log
servicelog-2021-02-22.20210222.8.log
servicelog-2021-02-22.20210222.7.log
servicelog-2021-02-22.20210222.6.log
servicelog-2021-02-22.20210222.5.log
servicelog-2021-02-22.20210222.4.log
servicelog-2021-02-22.20210222.3.log
servicelog-2021-02-22.20210222.2.log
servicelog-2021-02-22.20210222.1.log
Code language: plaintext (plaintext)