The logback.xml configuration snippet:
<property name="RSF_USER_HOME" value="/opt/logs/rsf"/>

<appender name="rsfStatistic" class="ch.qos.logback.core.rolling.RollingFileAppender">
<File>${RSF_USER_HOME}/Server1/ssds/rsf_statistic.log</File>

<rollingPolicy class="ch.qos.logback.core.rolling.FixedWindowRollingPolicy">
<FileNamePattern>${RSF_USER_HOME}/Server1/ssds/rsf_statistic.log.%i.bak</FileNamePattern>
<MinIndex>1</MinIndex>
<MaxIndex>5</MaxIndex>
</rollingPolicy>

<triggeringPolicy class="ch.qos.logback.core.rolling.SizeBasedTriggeringPolicy">
<MaxFileSize>100MB</MaxFileSize>
</triggeringPolicy>

<layout class="ch.qos.logback.classic.PatternLayout">
<Pattern>%msg%n</Pattern>
</layout>
</appender>

<logger name="rsf.statistic" level="INFO" additivity="false">
<appender-ref ref="rsfStatistic"/>
</logger>

The log file list:

You'll find that some log will still be appended to the rsf_statistic.log.1.bak which has already rolled over, simultaneously some different log will be appended to the current active file rsf_statistic.log. This bug is hard to reproduce deliberately, but occur in our several application system. It'll recover after restarting the application server. (Only one JVM is writing the file.)