NAME
systemd.offline-updates - Implementation of offline updates in systemdIMPLEMENTING OFFLINE SYSTEM UPDATES
This man page describes how to implement "offline" system updates with systemd. By "offline" OS updates we mean package installations and updates that are run with the system booted into a special system update mode, in order to avoid problems related to conflicts of libraries and services that are currently running with those on disk. This document is inspired by this GNOME design whiteboard[1]. The logic: 1.The package manager prepares system updates
by downloading all (.rpm or .deb or whatever) packages to update off-line in a
special directory /var/lib/system-update (or another directory of the
package/upgrade manager's choice).
2.When the user OK'ed the update, the symlink
/system-update is created that points to /var/lib/system-update (or wherever
the directory with the upgrade files is located) and the system is rebooted.
This symlink is in the root directory, since we need to check for it very
early at boot, at a time where /var/ is not available yet.
3.Very early in the new boot
systemd-system-update-generator(8) checks whether /system-update
exists. If so, it (temporarily and for this boot only) redirects (i.e.
symlinks) default.target to system-update.target, a special target that pulls
in the base system (i.e. sysinit.target, so that all file systems are mounted
but little else) and the system update units.
4.The system now continues to boot into
default.target, and thus into system-update.target. This target pulls in all
system update units. Only one service should perform an update (see the next
point), and all the other ones should exit cleanly with a "success"
return code and without doing anything. Update services should be ordered
after sysinit.target so that the update starts after all file systems have
been mounted.
5.As the first step, an update service should
check if the /system-update symlink points to the location used by that update
service. In case it does not exist or points to a different location, the
service must exit without error. It is possible for multiple update services
to be installed, and for multiple update services to be launched in parallel,
and only the one that corresponds to the tool that created the symlink
before reboot should perform any actions. It is unsafe to run multiple updates
in parallel.
6.The update service should now do its job.
If applicable and possible, it should create a file system snapshot, then
install all packages. After completion (regardless whether the update
succeeded or failed) the machine must be rebooted, for example by calling
systemctl reboot. In addition, on failure the script should revert to
the old file system snapshot (without the symlink).
7.The update scripts should exit only after
the update is finished. It is expected that the service which performs the
update will cause the machine to reboot after it is done. If the
system-update.target is successfully reached, i.e. all update services have
run, and the /system-update symlink still exists, it will be removed and the
machine rebooted as a safety measure.
8.After a reboot, now that the /system-update
symlink is gone, the generator won't redirect default.target anymore and the
system now boots into the default target again.
RECOMMENDATIONS
1.To make things a bit more robust we
recommend hooking the update script into system-update.target via a .wants/
symlink in the distribution package, rather than depending on systemctl
enable in the postinst scriptlets of your package. More specifically, for
your update script create a .service file, without [Install] section, and then
add a symlink like
/lib/systemd/system/system-update.target.wants/foobar.service →
../foobar.service to your package.
2.Make sure to remove the /system-update
symlink as early as possible in the update script to avoid reboot loops in
case the update fails.
3.Use FailureAction=reboot in the
service file for your update script to ensure that a reboot is automatically
triggered if the update fails. FailureAction= makes sure that the
specified unit is activated if your script exits uncleanly (by non-zero error
code, or signal/coredump). If your script succeeds you should trigger the
reboot in your own code, for example by invoking logind's Reboot() call
or calling systemctl reboot. See org.freedesktop.login1(5) for
details about the logind D-Bus API.
4.The update service should declare
DefaultDependencies=no, Requires=sysinit.target,
After=sysinit.target, After=system-update-pre.target,
Before=system-update.target and explicitly pull in any other services
it requires.
5.It may be desirable to always run an
auxiliary unit when booting into offline-updates mode, which itself does not
install updates. To do this create a .service file with
Wants=system-update-pre.target and
Before=system-update-pre.target and add a symlink to that file under
/lib/systemd/system-update.target.wants .
SEE ALSO
systemd(1), systemd.generator(7), systemd-system-update-generator(8), dnf.plugin.system-upgrade(8)NOTES
- 1.
- GNOME design whiteboard
systemd 252 |