Docker: Disk Devourer

My laptop’s disk was mysteriously filling up: turns out it was docker.

What Happened?

When your disk fills up, things get weird and your computer starts to whine. I was getting alerts saying my disk was nearly full, and ignored them until I literally could not work anymore: Git started to break down with the error fatal: Unable to write new index file. Thus blocked, and having no idea what could be consuming my laptop’s disk space, I ran this command to find out:

sudo du -h / | sort --human-numeric-sort --reverse | head

After a short while the results came back and fortunately the culprit was obvious: docker volumes. I hadn’t been cleaning up after myself when using docker all day every day.

205G  /
195G  /var
157G  /var/lib
153G  /var/lib/docker
100G  /var/lib/docker/volumes
53G   /var/lib/docker/overlay2
...

How I Solved It

Docker volumes will over time consume a lot of disk and don’t necessarily get cleaned up when you remove images or containers. With a little peek at the docs I found I could run this command to clear out all that docker volume cruft:

docker system prune -f --volumes

However, that command only does the cleanup once, merely extending the clock on a ticking time-bomb of disk-devouring. So, my simple fix was to add a cronjob to run this cleanup command once a week. In other words, I ran crontab -e and add the following lines (with the correct email, of course).

SHELL=/bin/bash
MAILTO=myname@email.com

# Prune unused docker cruft once a week.
0 2 * * 1 docker system prune -af --volumes

So, cron does the cleanup and sends me an email at around 2AM every Monday detailing what it cleaned up. Note that I added the -a option which also deletes images not tied to at least one container. In my case I have a decently-fast internet connection and regularly pull new images anyway, so I actually prefer this to keep the cruft to the absolute minimum. It has been a few months and this solution has worked brilliantly.