Reclaiming Docker Disc Space on OSX

Sep 6, 2017

A few nights ago, after a truly horrible night's sleep complete with my first ever incidence of acid reflux, I woke to find my Mac nattering at me about being out of disc space. And by out of disc space I mean that I was down to about 3 gigs out of a terabyte sized SSD. Yikes!

Taking a look at the normal culprits yielded no surprise increases and I can distinctly recall having in excess of 30 gigs just a day or two ago. Given all the server side problems with respect to Docker and disc space that I have written about previously, I had a strong hunch that somehow Docker was involved.

My first thought was that perhaps Docker logs are huge. On OSX, Docker logs are stored here:

~/Library/Containers/com.docker.docker/Data/com.docker.driver.amd64-linux/log

But a quick:

09:41 $ du -shc ~/Library/Containers/com.docker.docker/Data/com.docker.driver.amd64-linux/log
 20K	/Users/sjohnson/Library/Containers/com.docker.docker/Data/com.docker.driver.amd64-linux/log
 20K	total

Nope – clearly 20K isn't anything for disc space usage.

Containers

My next thought was that perhaps Docker has some massive backing store for containers. I did a bunch of searches and ran across this Docker thread and then another thread, both of which pointed me to the Docker.qcow2 file.

Here's what a du -shc showed me on this file:

du -shc     ~/Library/Containers/com.docker.docker/Data/com.docker.driver.amd64-linux/Docker.qcow2
 57G	/Users/sjohnson/Library/Containers/com.docker.docker/Data/com.docker.driver.amd64-linux/Docker.qcow2
 57G	total

Futile Attempts to Reclaim Space

I tried the normal things like:

docker system prune

And when that failed, I tried a bash based approach:

#!/bin/bash
for item in $(docker ps -aq); do
docker stop $item
docker rm $item
done

for item in $(docker images --filter dangling=true -q); do
docker rmi $item
done

But even after both of these, I still had:

du -shc     ~/Library/Containers/com.docker.docker/Data/com.docker.driver.amd64-linux/Docker.qcow2
 57G	total

My Solution

Here was the approach that I used:

1. Quit Docker for Mac entirely.
2. rm /Users/sjohnson/Library/Containers/com.docker.docker/Data/com.docker.driver.amd64-linux/Docker.qcow2
3. As best as I can tell this is actually safe because containers can always be rebuilt.
4. Restart Docker
5. Verify space usage:

du -shc ~/Library/Containers/com.docker.docker/Data/com.docker.driver.amd64-linux/Docker.qcow2 1.3G total

I then built a container and then reverified disc space usage.

du -shc     ~/Library/Containers/com.docker.docker/Data/com.docker.driver.amd64-linux/Docker.qcow2
1.3G	total

I then built some more containers and then checked disc space usage:

du -shc     ~/Library/Containers/com.docker.docker/Data/com.docker.driver.amd64-linux/Docker.qcow2
3.0G	total

I then started my overall "build all containers" shell script and tracked progress as it went:

du -shc     ~/Library/Containers/com.docker.docker/Data/com.docker.driver.amd64-linux/Docker.qcow2
3.0G	total

09:58 $ du -shc     ~/Library/Containers/com.docker.docker/Data/com.docker.driver.amd64-linux/Docker.qcow2
3.1G	total

du -shc     ~/Library/Containers/com.docker.docker/Data/com.docker.driver.amd64-linux/Docker.qcow2
3.4G    total

du -shc     ~/Library/Containers/com.docker.docker/Data/com.docker.driver.amd64-linux/Docker.qcow2
5.4G    total

By the time all containers were built:

du -shc     ~/Library/Containers/com.docker.docker/Data/com.docker.driver.amd64-linux/Docker.qcow2
7.2G    /Users/sjohnson/Library/Containers/com.docker.docker/Data/com.docker.driver.amd64-linux/Docker.qcow2

And now, several days later and even more deploys, it has increased again:

du -shc     ~/Library/Containers/com.docker.docker/Data/com.docker.driver.amd64-linux/Docker.qcow2
8.2G	/Users/sjohnson/Library/Containers/com.docker.docker/Data/com.docker.driver.amd64-linux/Docker.qcow2
8.2G	total

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