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Using systemd as a scheduler

Scheduling tasks on a personal computer is crucial for maintaining performance, performing regular maintenance tasks like backups or scrubs, and automating repetitive actions. While cron has been the traditional tool for task scheduling on Unix-like systems, systemd timers offer a more robust and flexible alternative, especially for systems that are not always running, like PCs. This blog post will explore how you can use systemd as a scheduler, including an example of setting up a weekly ZFS scrub using systemd timers.

Some background:

I have recently set up a ZFS pool (tutorial) for write-intensive applications, such as creating and destroying Docker containers, web scraping, or generating gigabytes of random images for model training. I want to schedule a weekly scrub, but since it’s a PC, cron won’t work reliably due to the variable on/off times of the computer. This is where I started looking for alternatives.

Why Use systemd Timers Over cron?​

systemd timers provide several advantages over cron:

  • Persistent Timers: If a computer is off when the timer is supposed to run, systemd can ensure the task is executed as soon as possible once the system is back online.
  • More Powerful Scheduling: systemd timers support a more powerful scheduling syntax, allowing for more complex scheduling scenarios.
  • Unified Management: Since many modern Linux distributions use systemd for service management, using systemd timers allows you to manage all services and scheduled tasks uniformly.

Setting Up a Weekly ZFS Scrub with systemd Timer​

ZFS scrubbing is a maintenance task that scans the storage pool for data integrity and repairs any inconsistencies. It is essential to run scrubs regularly, especially on systems using SSDs or other storage mediums that benefit from regular checks. In this example, we'll set up a weekly ZFS scrub using systemd.

1. Creating a systemd Service for ZFS Scrub​

First, we need to create a systemd service file that defines the ZFS scrub command. This service will be executed by our timer.

  1. Create a new service file:

    sudo nano /etc/systemd/system/zfs-scrub.service

  2. Add the following content to the file:

    [Unit]
    Description=ZFS scrub for all pools

    [Service]
    Type=oneshot
    ExecStart=/sbin/zpool scrub <pool_name>

    Replace <pool_name> with the name of your ZFS pool (e.g., mypool). If you have multiple pools, add multiple ExecStart lines:

    ExecStart=/sbin/zpool scrub pool1
    ExecStart=/sbin/zpool scrub pool2

  3. Save and exit the file.

2. Creating a systemd Timer for the Service​

Next, we create a systemd timer that will trigger the service on a weekly schedule.

  1. Create a new timer file:

    sudo nano /etc/systemd/system/zfs-scrub.timer

  2. Add the following content to the file:

    [Unit]
    Description=Weekly ZFS scrub

    [Timer]
    OnCalendar=weekly
    Persistent=true

    [Install]
    WantedBy=timers.target
    • OnCalendar=weekly: This schedules the timer to trigger once a week.
    • Persistent=true: Ensures that the scrub runs as soon as possible if the PC was off during the scheduled time.

  3. Save and exit the file.

3. Enabling and Starting the Timer​

  1. Reload systemd to recognize the new service and timer:

    sudo systemctl daemon-reload

  2. Enable the timer so that it starts on boot:

    sudo systemctl enable zfs-scrub.timer

  3. Start the timer immediately:

    sudo systemctl start zfs-scrub.timer

4. Verifying the Timer​

To verify that the timer is active and see when it will run next, use:

systemctl list-timers --all | grep zfs-scrub

You should see output indicating when the next run is scheduled.

Advantages of Using systemd Timers for Personal PCs​

For personal computers that may not be running 24/7, systemd timers provide a reliable way to ensure maintenance tasks are executed regularly:

  • Handles Missed Runs: Unlike cron, systemd timers with the Persistent=true setting will catch up on missed runs the next time the computer is powered on.
  • Easier Management: Using systemctl commands, you can start, stop, enable, disable, and inspect timers easily.
  • Unified Configuration: With systemd, both services and scheduled tasks are managed in a consistent manner.

Other Use Cases for systemd Timers​

In addition to ZFS scrubbing, you can use systemd timers to schedule various tasks:

  • Backups: Schedule regular backups with tools like rsync.
  • System Updates: Run system updates automatically on a regular basis.
  • Maintenance Scripts: Run custom maintenance scripts that clean up logs, rotate files, or perform other housekeeping tasks.

Conclusion​

systemd timers provide a powerful and flexible way to schedule tasks on your Linux system, especially for personal PCs that are not always on. By using systemd timers, you can ensure important maintenance tasks like ZFS scrubbing are performed regularly without worrying about whether the computer was on at the scheduled time. With its persistent and robust scheduling capabilities, systemd is an excellent choice for automating tasks and maintaining the health of your system.

By following the example provided, you can set up a weekly ZFS scrub to maintain your data integrity and keep your storage pool in top condition. This method can be extended to other maintenance tasks, making systemd a versatile scheduler for personal and professional use.

First published on 2024-08-14