# Kexec

Related articles

- Systemd

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 kexec may not work correctly for you due to devices not fully re-initializing when using this method, however this is rarely the case.

## Installation

Install the kexec-tools package.

## Rebooting using kexec

### Manually

You can manually invoke kexec using:

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

Warning: Running `kexec -e` directly will not unmount active filesystems or terminate any running services gracefully.

It is also possible to load kernel manually and then let systemd handle service shutdown and kexec for you:

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

### Systemd

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

    /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
    Requires=sysinit.target
    After=sysinit.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

Note that Conflicts=shutdown.target is not really needed, as it's implicitly guaranteed by strict ordering on systinit.target which itself Conflicts= with shutdown.target.

## See also

- [systemd-devel] Right way to do kexec
- kdump: a kexec based crash dumping mechansim for Linux
- Reboot Linux faster using kexec

Categories:

- Boot process
- Kernel