Difference between revisions of "Kexec"

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(Manually)
(Add a unit file that works when /boot is on a different partition)
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{{bc|# systemctl disable kexec-load@linux
 
{{bc|# systemctl disable kexec-load@linux
 
# systemctl enable kexec-load@linux-lts}}
 
# systemctl enable kexec-load@linux-lts}}
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 +
====Separate /boot partition====
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 +
The above systemd unit file will fail if /boot is not on the root file system, as systemd will likely unmount /boot before it runs the kexec-load unit file.  An alternative approach is to load a "hook" unit file that does nothing on startup and invokes kexec upon termination.  By making this unit file conflict with kexec.target and only kexec.target, you can ensure the new kernel gets loaded early enough and only after a "systemctl kexec" command.  Here is an alternate {{ic|/etc/systemd/system/kexec-load@.service}} file that follows this strategy:
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{{bc|1=
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[Unit]
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Description=hook to load vmlinuz-%i kernel upon kexec
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Documentation=man:kexec(8)
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DefaultDependencies=no
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RequiresMountsFor=/boot/vmlinuz-%i
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Conflicts=kexec.target
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 +
[Service]
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Type=oneshot
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ExecStart=-/usr/bin/true
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RemainAfterExit=yes
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ExecStop=/usr/bin/kexec -l /boot/vmlinuz-%i --initrd=/boot/initramfs-%i.img --reuse-cmdline
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 +
[Install]
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WantedBy=basic.target
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}}
  
 
===Manually===
 
===Manually===

Revision as of 19:59, 10 August 2013

Template:Article summary start Template:Article summary text Template:Article summary heading Template:Article summary wiki Template:Article summary end

Kexec is a system call that enables you to load and boot into another kernel from the currently running kernel. This is useful for kernel developers or other people who need to reboot very quickly without waiting for the whole BIOS boot process to finish. Note that there may appear some problems and kexec may not work correctly for you because the devices won't fully reinitiate when using this method.

Installation

To install kexec, install the kexec-tools package which is available in the official repositories.

Rebooting using kexec

Systemd

You will need to create a new unit file, kexec-load@.service, that will load the specified kernel to be kexec'ed[1].

# vim /etc/systemd/system/kexec-load@.service
[Unit]
Description=load %i kernel into the current kernel
Documentation=man:kexec(8)
DefaultDependencies=no
Before=shutdown.target umount.target final.target

[Service]
Type=oneshot
ExecStart=/usr/bin/kexec -l /boot/vmlinuz-%i --initrd=/boot/initramfs-%i.img --reuse-cmdline

[Install]
WantedBy=kexec.target

Then enable the service file for the kernel you want to load, for example simply the default kernel linux:

# systemctl enable kexec-load@linux
ln -s '/etc/systemd/system/kexec-load@.service' '/etc/systemd/system/kexec.target.wants/kexec-load@linux.service'

Ensure that the shutdown hook is not part of your initramfs image by removing it from the HOOKS array in /etc/mkinitcpio.conf. If it is, remove it and rebuild your initrd image with mkinitcpio -p linux.

Then to kexec

# systemctl kexec

If you wish to load a different kernel for the next kexec, for example linux-lts, disable the service for the current kernel and enable the one for the new kernel:

# systemctl disable kexec-load@linux
# systemctl enable kexec-load@linux-lts

Separate /boot partition

The above systemd unit file will fail if /boot is not on the root file system, as systemd will likely unmount /boot before it runs the kexec-load unit file. An alternative approach is to load a "hook" unit file that does nothing on startup and invokes kexec upon termination. By making this unit file conflict with kexec.target and only kexec.target, you can ensure the new kernel gets loaded early enough and only after a "systemctl kexec" command. Here is an alternate /etc/systemd/system/kexec-load@.service file that follows this strategy:

[Unit]
Description=hook to load vmlinuz-%i kernel upon kexec
Documentation=man:kexec(8)
DefaultDependencies=no
RequiresMountsFor=/boot/vmlinuz-%i
Conflicts=kexec.target

[Service]
Type=oneshot
ExecStart=-/usr/bin/true
RemainAfterExit=yes
ExecStop=/usr/bin/kexec -l /boot/vmlinuz-%i --initrd=/boot/initramfs-%i.img --reuse-cmdline

[Install]
WantedBy=basic.target

Manually

It is also perfectly legal to invoke kexec manually:

 # kexec -l /boot/vmlinuz-linux --initrd=/boot/initramfs-linux.img --reuse-cmdline
 # kexec -e

References

1. [systemd-devel] Right way to do kexec

See also