Difference between revisions of "Kexec"

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kernel from the currently running kernel. This is useful for kernel developers
 
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
 
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
+
BIOS boot process to finish. Note that kexec may not work correctly for you
may not work correctly for you because the devices ''won't'' fully reinitiate
+
due to devices ''not'' fully re-initializing when using this method, however
when using this method.
+
this is rarely the case.
  
 
==Installation==
 
==Installation==

Revision as of 03:41, 21 December 2013

Summary help replacing me
Covers how to install and configure kexec.
Related
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

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

Unfortunately, while the above file works reliably on some machines, the ExecStop command seems to fail to run on others. Hence, at present there does not appear to be a known reliable way to make "systemctl kexec" do the right thing on all machines. Until then, it may be best to call kexec manually before invoking "systemctl kexec" (or even "reboot", which will do the right thing when a kernel is loaded).

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