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I want to investigate why my machine suddenly rebooted but I am not finding the core file.

My machine has RH 7 and kdump installed:

rpm -qa | grep kexec-tools
kexec-tools-2.0.7-19.el7_1.2.x86_64

However, I cant find the core file under /var

  find /var -name core

Is it possible that the core file was not created regarding the reboot problem?

And why was the core file not created under /var in spite of the kdump service being installed?

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kernel core file are created upon a kernel panic. That is some code in kernel goes wrong (common faults are a division by 0 and index out of bound in an array). User space program will generate core on same condition. However, not all reboot will generate core.

If you can afford to reboot your host, you might try

echo b > /proc/sysrq-trigger

this will reboot the host in no time, without any usefull comment on any log files.

Knowing why a host rebooted is a long and uncertain quest. Start by searching /var/log's files.

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