FAQs for Docker Desktop for Linux

FAQ
Table of contents

Why does Docker Desktop for Linux run a VM?

Docker Desktop for Linux runs a Virtual Machine (VM) for the following reasons:

  1. To ensure that Docker Desktop provides a consistent experience across platforms.

    During research, the most frequently cited reason for users wanting Docker Desktop for Linux was to ensure a consistent Docker Desktop experience with feature parity across all major operating systems. Utilizing a VM ensures that the Docker Desktop experience for Linux users will closely match that of Windows and macOS.

  2. To make use of new kernel features.

    Sometimes we want to make use of new operating system features. Because we control the kernel and the OS inside the VM, we can roll these out to all users immediately, even to users who are intentionally sticking on an LTS version of their machine OS.

  3. To enhance security.

    Container image vulnerabilities pose a security risk for the host environment. There is a large number of unofficial images that are not guaranteed to be verified for known vulnerabilities. Malicious users can push images to public registries and use different methods to trick users into pulling and running them. The VM approach mitigates this threat as any malware that gains root privileges is restricted to the VM environment without access to the host.

    Why not run rootless Docker? Although this has the benefit of superficially limiting access to the root user so everything looks safer in "top", it allows unprivileged users to gain CAP_SYS_ADMIN in their own user namespace and access kernel APIs which are not expecting to be used by unprivileged users, resulting in vulnerabilities.

  4. To provide the benefits of feature parity and enhanced security, with minimal impact on performance.

    The VM utilized by Docker Desktop for Linux uses VirtioFS, a shared file system that allows virtual machines to access a directory tree located on the host. Our internal benchmarking shows that with the right resource allocation to the VM, near native file system performance can be achieved with VirtioFS.

    As such, we have adjusted the default memory available to the VM in Docker Desktop for Linux. You can tweak this setting to your specific needs by using the Memory slider within the Settings > Resources tab of Docker Desktop.

How do I enable file sharing?

Docker Desktop for Linux uses VirtioFS as the default (and currently only) mechanism to enable file sharing between the host and Docker Desktop VM.

In order not to require elevated privileges, without unnecessarily restricting operations on the shared files, Docker Desktop runs the file sharing service (virtiofsd) inside a user namespace (see user_namespaces(7)) with UID and GID mapping configured. As a result Docker Desktop relies on the host being configured to enable the current user to use subordinate ID delegation. For this to be true /etc/subuid (see subuid(5)) and /etc/subgid (see subgid(5)) must be present. Docker Desktop only supports subordinate ID delegation configured via files. Docker Desktop maps the current user ID and GID to 0 in the containers. It uses the first entry corresponding to the current user in /etc/subuid and /etc/subgid to set up mappings for IDs above 0 in the containers.

ID in containerID on host
0 (root)ID of the user running DD (e.g. 1000)
10 + beginning of ID range specified in /etc/subuid//etc/subgid (e.g. 100000)
21 + beginning of ID range specified in /etc/subuid//etc/subgid (e.g. 100001)
32 + beginning of ID range specified in /etc/subuid//etc/subgid (e.g. 100002)
......

If /etc/subuid and /etc/subgid are missing, they need to be created. Both should contain entries in the form - <username>:<start of id range>:<id range size>. For example, to allow the current user to use IDs from 100 000 to 165 535:

$ grep "$USER" /etc/subuid >> /dev/null 2&>1 || (echo "$USER:100000:65536" | sudo tee -a /etc/subuid)
$ grep "$USER" /etc/subgid >> /dev/null 2&>1 || (echo "$USER:100000:65536" | sudo tee -a /etc/subgid)

To verify the configs have been created correctly, inspect their contents:

$ echo $USER
exampleuser
$ cat /etc/subuid
exampleuser:100000:65536
$ cat /etc/subgid
exampleuser:100000:65536

In this scenario if a shared file is chowned inside a Docker Desktop container owned by a user with a UID of 1000, it shows up on the host as owned by a user with a UID of 100999. This has the unfortunate side effect of preventing easy access to such a file on the host. The problem is resolved by creating a group with the new GID and adding our user to it, or by setting a recursive ACL (see setfacl(1)) for folders shared with the Docker Desktop VM.

Where does Docker Desktop store Linux containers?

Docker Desktop stores Linux containers and images in a single, large "disk image" file in the Linux filesystem. This is different from Docker on Linux, which usually stores containers and images in the /var/lib/docker directory on the host's filesystem.

Where is the disk image file?

To locate the disk image file, select Settings from the Docker Desktop Dashboard then Advanced from the Resources tab.

The Advanced tab displays the location of the disk image. It also displays the maximum size of the disk image and the actual space the disk image is consuming. Note that other tools might display space usage of the file in terms of the maximum file size, and not the actual file size.

What if the file is too large?

If the disk image file is too large, you can:

  • Move it to a bigger drive
  • Delete unnecessary containers and images
  • Reduce the maximum allowable size of the file
How do I move the file to a bigger drive?

To move the disk image file to a different location:

  1. Select Settings then Advanced from the Resources tab.

  2. In the Disk image location section, select Browse and choose a new location for the disk image.

  3. Select Apply & Restart for the changes to take effect.

Do not move the file directly in Finder as this can cause Docker Desktop to lose track of the file.

How do I delete unnecessary containers and images?

Check whether you have any unnecessary containers and images. If your client and daemon API are running version 1.25 or later (use the docker version command on the client to check your client and daemon API versions), you can see the detailed space usage information by running:

$ docker system df -v

Alternatively, to list images, run:

$ docker image ls

To list containers, run:

$ docker container ls -a

If there are lots of redundant objects, run the command:

$ docker system prune

This command removes all stopped containers, unused networks, dangling images, and build cache.

It might take a few minutes to reclaim space on the host depending on the format of the disk image file:

  • If the file is named Docker.raw: space on the host should be reclaimed within a few seconds.
  • If the file is named Docker.qcow2: space will be freed by a background process after a few minutes.

Space is only freed when images are deleted. Space is not freed automatically when files are deleted inside running containers. To trigger a space reclamation at any point, run the command:

$ docker run --privileged --pid=host docker/desktop-reclaim-space

Note that many tools report the maximum file size, not the actual file size. To query the actual size of the file on the host from a terminal, run:

$ cd ~/.docker/desktop/vms/0/data
$ ls -klsh Docker.raw
2333548 -rw-r--r--@ 1 username  staff    64G Dec 13 17:42 Docker.raw

In this example, the actual size of the disk is 2333548 KB, whereas the maximum size of the disk is 64 GB.

How do I reduce the maximum size of the file?

To reduce the maximum size of the disk image file:

  1. From Docker Desktop Dashboard select Settings then Advanced from the Resources tab.

  2. The Disk image size section contains a slider that allows you to change the maximum size of the disk image. Adjust the slider to set a lower limit.

  3. Select Apply & Restart.

When you reduce the maximum size, the current disk image file is deleted, and therefore, all containers and images are lost.