NLog – Archive by file size

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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.logCode 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 archive 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.logCode language: plaintext (plaintext)

Note: When you’re archiving by size and have enabled trace logging, you may want to split the trace log into its own files with its own archive settings. Otherwise the trace logging will quickly fill up the logs.

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