Reclaim disk space after removing file from Docker container

Question

I've successfully copied a large database backup file (35 GB) into the Docker container and restored my database locally (following this walkthrough). I want to now delete that .bak file from the Docker container to reclaim the space. I did that by running sudo docker exec sql_server rm -rf /var/opt/mssql/backup/example.bak but this didn’t reclaim the space - my Docker.raw file remains about 76 GB. When I run docker system df it says my containers are 45 GB though. I tried docker system prune -a but this reclaimed 0B. Restarting Docker didn't do the trick. How do I shrink that now that the file is removed in order to gain that space back?

Answer 1

I did that by running sudo docker exec sql_server rm -rf /var/opt/mssql/backup/example.bak but this didn’t reclaim the space

Whether this will free up space depends on whether the file exists only in the container or if it exists in your image. Once a file exists in the image, deleting it in the container doesn't modify the image itself. Instead only the container filesystem is updated with an indication that the file is deleted from the view of that container. This is how the layered filesystem works under the covers.

When I run docker system df it says my containers are 45 GB though

You can examine this a bit deeper. For any specific container, you can run a docker container diff command on the container id to see the files that have been modified inside that container.

I tried docker system prune -a but this reclaimed 0B.

This will not reclaim space from a running container. If the container is stopped, it will be deleted, and the image that started that container may also be deleted if nothing else points to it. Otherwise docker will avoid running containers and there's no pruning it can run on the files inside a running container.

my Docker.raw file remains about 76 GB

This is a very key point, it suggests that you are running Docker on a Mac. All of the above steps may reduce disk space of the Linux environment that Docker runs on top of. However, the VM that Docker uses on Mac and Windows is mapped to a file that grows on demand as the VM needs it. From the Docker for Mac FAQ, diskspace reported by this file may not be accurate because of sparse files work on Mac:

Docker.raw consumes an insane amount of disk space!

This is an illusion. Docker uses the raw format on Macs running the Apple Filesystem (APFS). APFS supports sparse files, which compress long runs of zeroes representing unused space. The output of ls is misleading, because it lists the logical size of the file rather than its physical size. To see the physical size, add the -ks switch; to see the logical size in human readable form, add -lh:

$ cd ~/Library/Containers/com.docker.docker/Data/vms/0
$ ls -klsh Docker.raw
2333548 -rw-r--r--@ 1 akim  staff    64G Dec 13 17:42  Docker.raw

In this listing, the logical size is 64GB, but the physical size is only 2.3GB.

Alternatively, you may use du (disk usage):

$ du -h Docker.raw
2,2G  Docker.raw

I'd also recommend looking at how much disk space is used inside the Docker VM with:

sudo docker run --rm -v /var/lib/docker:/host/var/lib/docker:ro \
  busybox df -h /host/var/lib/docker