What is Docker.qcow2?

Question

I was looking at my disk with DaisyDisk and I have a 30GB something called Docker.qcow2. More specifically, DaisyDisk puts it under ~/Library/Containers/com.docker.doc

Answer 1

The .qcow2 file is exposed to the VM as a block device with a maximum size of 64GiB by default. As new files are created in the filesystem by containers, new sectors are written to the block device. These new sectors are appended to the .qcow2 file causing it to grow in size, until it eventually becomes fully allocated. It stops growing when it hits this maximum size.

You can stop Docker and delete this file, however deleting it will also remove all your containers and images. And Docker will recreate this file on start.

Answer 2

If you stumbled upon this, you're probably not stoked about a 64gb file. If you open up Docker > Preferences, you can tone it down quite a bit to a more reasonable size. Doing this will delete the old cow file, and that will delete your containers, so be careful.