为什么 systemctl status 输出 "Warning:Journal has been rotated since unit was started.Log output is incomplete or unavailable"?

Issue

在通过 systemctl status 检查状态时，我可以看到以下警告消息

Warning: Journal has been rotated since unit was started. Log output is incomplete or unavailable.

这意味着什么？
为什么显示此消息？
我应该怎么处理此消息？

即

# systemctl status systemd-journald

● systemd-journald.service - Journal Service
   Loaded: loaded (/usr/lib/systemd/system/systemd-journald.service; static; vendor preset: disabled)
   Active: active (running) since 2019-03-28 10:49:35 JST; 5 months 16 days ago
     Docs: man:systemd-journald.service(8)
           man:journald.conf(5)
 Main PID: 2616 (systemd-journal)
   Status: "Processing requests..."
   CGroup: /system.slice/systemd-journald.service
           mq2616 /usr/lib/systemd/systemd-journald

Warning: Journal has been rotated since unit was started. Log output is incomplete or unavailable.