View the dmesg log before server restart

notes : The sample code in this article is Ubuntu Execute... In the system , other Linux The distribution operation is similar .

Scene description

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In the daily operation and maintenance work , Inevitably, we will encounter the problem of server failure . Although in application operation and maintenance or SRE post , You don't have to repair the server yourself , But it is necessary to investigate the cause of the problem , In order to submit the repair order later .

In various types of server failures , One kind is like this ：

  One day , The server suddenly fails to log in , After the server restarts, it returns to normal .

After the server restarts and returns to normal , We log in to the server , It is necessary to judge whether the fault is caused by hardware problems , If there is a hardware problem , Usually in dmesg It is reflected in the log .

The problem is , After every server restart , Log in to the machine and execute dmesg command , Can only show the log after this restart , You can't see the log before restarting . In order to find out the cause of server failure , You must see the before restart dmesg journal .

Ubuntu < 15.04

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Now use less than Ubuntu 15.04 There should be few people in this version , But if you are using these old Ubuntu System words , In your /var/log/ Under the table of contents , There will be a log file with the following name ：

dmesg
dmesg.0
dmesg.1.gz
...

dmesg What is saved in the log file is the log after this restart , and dmesg.0 It was before this restart dmesg journal ,dmesg.1.gz It was before the last restart dmesg journal , By analogy .

By looking at dmesg.0 The contents of the log file , You can see the before the server restarts dmesg Log .

Ubuntu >= 15.04

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Now more people use Ubuntu Should be 15.04 Later versions .Ubuntu from 15.04 Start using systemd, systemd There is a logging service in , be known as journald, Through command line tools journalctl It's easy to get journald Recorded log , This includes dmesg journal .

To view the before server restart dmesg journal , You can use the following command ：

journalctl -k -b -1

-k: Indicates to view dmesg journal

-b -1:  It means to check the log before the server restarts

Conclusion

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When troubleshooting the server ,dmesg Logs are a very useful source of information . If you're using Ubuntu 15.04 Previous systems ,/var/log/dmesg.0 Before restart dmesg journal ; If you use it Ubuntu 15.04 After the system , adopt journalctl Command you can easily view the information before the server restarts dmesg journal .

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