I found that CentOS/Redhat will not store systemd-journal log to disk, Ubuntu will.
For CentOS/Redhat, each time I reboot, the journal log were removed ( it seems write to tmpfs only )
Why CentOS/Redhat don't want to keep journal log?
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Possible duplicate, see: How do I display log messages from previous boots under CentOS 7?
By default, the journal stores log data in /run/log/journal/. Since /run/ is volatile, log data is lost at reboot. To make the data persistent, it is sufficient to create /var/log/journal/ where systemd-journald will then store the data.