■kdump設定
①systemctl start kdump.serviceで起動した際にエラーになる
②cat /var/log/messages | grep kdump
Apr 23 07:05:51 localhost systemd: Unit kdump.service entered failed state.
Apr 23 07:05:51 localhost systemd: kdump.service failed.
Apr 23 07:09:11 localhost kdumpctl: No memory reserved for crash kernel.
Apr 23 07:09:11 localhost kdumpctl: Starting kdump: [FAILED]
Apr 23 07:09:11 localhost systemd: kdump.service: main process exited, code=exited, status=1/FAILURE

No memory reserved for crash kernel.
カーネルクラッシュ時のメモリがないよ。という意味らしい

③/etc/default/grubを編集
GRUB_CMDLINE_LINUX="crashkernel=auto rhgb quiet"
↓
GRUB_CMDLINE_LINUX="rd.lvm.lv=centos/swap vconsole.font=latarcyrheb-sun16 vconsole.keymap=jp106 rd.lvm.lv=centos/root crashkernel=128M rhgb quiet"

④上記のように crashkernel=auto → crashkernel=128M へ変更
編集後、grub2-mkconfig で、最新のgrub設定ファイルを生成
grub2-mkconfig -o /boot/grub2/grub.cfg

⑤生成後、リブート
reboot

⑥再度、kdumpの状態を確認
[root@localhost ~]# systemctl status kdump.service
● kdump.service - Crash recovery kernel arming
Loaded: loaded (/usr/lib/systemd/system/kdump.service; enabled; vendor preset: enabled)
Active: active (exited) since Tue 2019-04-23 08:06:36 PDT; 1min 23s ago
Process: 4398 ExecStart=/usr/bin/kdumpctl start (code=exited, status=0/SUCCESS)
Main PID: 4398 (code=exited, status=0/SUCCESS)
Tasks: 0
CGroup: /system.slice/kdump.service

Apr 23 08:06:27 localhost.localdomain systemd[1]: Starting Crash recovery ker...
Apr 23 08:06:36 localhost.localdomain kdumpctl[4398]: kexec: loaded kdump kernel
Apr 23 08:06:36 localhost.localdomain kdumpctl[4398]: Starting kdump: [OK]
Apr 23 08:06:36 localhost.localdomain systemd[1]: Started Crash recovery kern...
Hint: Some lines were ellipsized, use -l to show in full.

⑦/var/log/messagesでも、問題ないことを確認する。
[root@localhost ~]# cat /var/log/messages | grep kdump
Apr 23 08:06:36 localhost kdumpctl: kexec: loaded kdump kernel
Apr 23 08:06:36 localhost kdumpctl: Starting kdump: [OK]