[systemd-devel] Hints for upgrading systemd on a running system

[Paul Menzel]

Dear systemd folks,


We finally are going to upgrade from a very old systemd version 27 from 
2011 to the current systemd v237. (Historical reasons.)

Anyway, I already was told about `systemctl daemon-reexec`, and we got 
it working.

After that, looking at the output of `systemctl`, there are many units 
from the old version, which were removed in the meantime.

```
$ systemctl --state=not-found
   UNIT                                 LOAD      ACTIVE   SUB 
DESCRIPTION
● dev-hugepages.automount              not-found active   waiting 
dev-hugepages.automount
● dev-mqueue.automount                 not-found active   waiting 
dev-mqueue.automount
● sys-kernel-debug.automount           not-found active   waiting 
sys-kernel-debug.automount
● sys-kernel-security.automount        not-found active   waiting 
sys-kernel-security.automount
● auditd.service                       not-found inactive dead 
auditd.service
● console-kit-log-system-start.service not-found active   exited 
console-kit-log-system-start.service
● display-manager.service              not-found inactive dead 
display-manager.service
● hwclock-load.service                 not-found active   exited 
hwclock-load.service
● plymouth-quit-wait.service           not-found inactive dead 
plymouth-quit-wait.service
● plymouth-start.service               not-found inactive dead 
plymouth-start.service
● remount-rootfs.service               not-found active   exited 
remount-rootfs.service
● syslog.service                       not-found inactive dead 
syslog.service
● systemd-kmsg-syslogd.service         not-found active   running 
systemd-kmsg-syslogd.service
● systemd-remount-api-vfs.service      not-found active   exited 
systemd-remount-api-vfs.service
● systemd-sysusers.service             not-found inactive dead 
systemd-sysusers.service
● udev-retry.service                   not-found active   exited 
udev-retry.service
● udev-settle.service                  not-found active   exited 
udev-settle.service
● systemd-logger.socket                not-found active   listening 
systemd-logger.socket
● systemd-shutdownd.socket             not-found active   listening 
systemd-shutdownd.socket
● cryptsetup.target                    not-found active   active 
cryptsetup.target
● syslog.target                        not-found active   active 
syslog.target

LOAD   = Reflects whether the unit definition was properly loaded.
ACTIVE = The high-level unit activation state, i.e. generalization of SUB.
SUB    = The low-level unit activation state, values depend on unit type.

21 loaded units listed. Pass --all to see loaded but inactive units, too.
To show all installed unit files use 'systemctl list-unit-files'.
```

Do I need to stop those manually beforehand, or is there another way to 
clean up?

Is the recommended update procedure documented somewhere?


Kind regards,

Paul