Tookmund

Uptime is often considered a measure of system reliability, an indication that the running software is stable and can be counted on.

However, this hides the insidious build-up of state throughout the system as it runs, the slow drift from the expected to the strange.

As Nolan Lawson highlights in an excellent post entitled Programmers are bad at managing state, state is the most challenging part of programming. It’s why “did you try turning it off and on again” is a classic tech support response to any problem.

You: uptime

Me: Every machine gets rebooted at 1AM to clear the slate for maintenance, and at 3:30AM to push through any pending updates.

— @SwiftOnSecurity, December 27, 2020

In addition to the problem of state, installing regular updates periodically requires a reboot, even if the rest of the process is automated through a tool like unattended-upgrades.

For my personal homelab, I manage a handful of different machines running various services.

I used to just schedule a day to update and reboot all of them, but that got very tedious very quickly.

I then moved the reboot to a cronjob, and then recently to a systemd timer and service.

I figure that laying out my path to better management of this might help others, and will almost certainly lead to someone telling me a better way to do this.

UPDATE: Turns out there’s another option for better systemd cron integration. See systemd-cron below.

Ultimately, uptime only measures the duration since you last proved you can turn the machine on and have it boot.

— @SwiftOnSecurity, May 7, 2016

Stage One: Reboot Cron

The first, and easiest approach, is a simple cron job. Just adding the following line to /var/spool/cron/crontabs/root¹ is enough to get your machine to reboot once a month² on the 6th at 8:00 AM³:

0 8 6 * * reboot

I had this configured for many years and it works well. But you have no indication as to whether it succeeds except for checking your uptime regularly yourself.

Stage Two: Reboot systemd Timer

The next evolution of this approach for me was to use a systemd timer. I created a regular-reboot.timer with the following contents:

[Unit]
Description=Reboot on a Regular Basis

[Timer]
Unit=regular-reboot.service
OnBootSec=1month

[Install]
WantedBy=timers.target

This timer will trigger the regular-reboot.service systemd unit when the system reaches one month of uptime.

I’ve seen some guides to creating timer units recommend adding a Wants=regular-reboot.service to the [Unit] section, but this has the consequence of running that service every time it starts the timer. In this case that will just reboot your system on startup which is not what you want.

Care needs to be taken to use the OnBootSec directive instead of OnCalendar or any of the other time specifications, as your system could reboot, discover its still within the expected window and reboot again. With OnBootSec your system will not have that problem. Technically, this same problem could have occurred with the cronjob approach, but in practice it never did, as the systems took long enough to come back up that they were no longer within the expected window for the job.

I then added the regular-reboot.service:

[Unit]
Description=Reboot on a Regular Basis
Wants=regular-reboot.timer

[Service]
Type=oneshot
ExecStart=shutdown -r 02:45

You’ll note that this service is actually scheduling a specific reboot time via the shutdown command instead of just immediately rebooting. This is a bit of a hack needed because I can’t control when the timer runs exactly when using OnBootSec. This way different systems have different reboot times so that everything doesn’t just reboot and fail all at once. Were something to fail to come back up I would have some time to fix it, as each machine has a few hours between scheduled reboots.

One you have both files in place, you’ll simply need to reload configuration and then enable and start the timer unit:

systemctl daemon-reload
systemctl enable --now regular-reboot.timer

You can then check when it will fire next:

# systemctl status regular-reboot.timer
● regular-reboot.timer - Reboot on a Regular Basis
     Loaded: loaded (/etc/systemd/system/regular-reboot.timer; enabled; preset: enabled)
     Active: active (waiting) since Wed 2024-03-13 01:54:52 EDT; 1 week 4 days ago
    Trigger: Fri 2024-04-12 12:24:42 EDT; 2 weeks 4 days left
   Triggers: ● regular-reboot.service

Mar 13 01:54:52 dorfl systemd[1]: Started regular-reboot.timer - Reboot on a Regular Basis.

Sidenote: Replacing all Cron Jobs with systemd Timers

More generally, I’ve now replaced all cronjobs on my personal systems with systemd timer units, mostly because I can now actually track failures via prometheus-node-exporter. There are plenty of ways to hack in cron support to the node exporter, but just moving to systemd units provides both support for tracking failure and logging, both of which make system administration much easier when things inevitably go wrong.

systemd-cron

An alternative to converting everything by hand, if you happen to have a lot of cronjobs is systemd-cron. It will make each crontab and /etc/cron.* directory into automatic service and timer units.

Thanks to Alexandre Detiste for letting me know about this project. I have few enough cron jobs that I’ve already converted, but for anyone looking at a large number of jobs to convert you’ll want to check it out!

Stage Three: Monitor that it’s working

The final step here is confirm that these units actually work, beyond just firing regularly.

I now have the following rule in my prometheus-alertmanager rules:

  - alert: UptimeTooHigh
    expr: (time() - node_boot_time_seconds{job="node"}) / 86400 > 35
    annotations:
      summary: "Instance  Has Been Up Too Long!"
      description: "Instance  Has Been Up Too Long!"

This will trigger an alert anytime that I have a machine up for more than 35 days. This actually helped me track down one machine that I had forgotten to set up this new unit on⁴.

Not everything needs to scale

One of the most common fallacies programmers fall into is that we will jump to automating a solution before we stop and figure out how much time it would even save.

In taking a slow improvement route to solve this problem for myself, I’ve managed not to invest too much time⁵ in worrying about this but also achieved a meaningful improvement beyond my first approach of doing it all by hand.