Beginning with Docker..

  1. To get the basic information about your Docker configuration, execute:

    $ docker info

    The out put will be as which contains information about the storage driver and the docker root directory. Storage Driver: overlay2 Docker Root Dir: /var/lib/docker

  2. To get the storage location of the docker images and containers.

  • Ubuntu: /var/lib/docker/
  • Fedora: /var/lib/docker/
  • Debian: /var/lib/docker/
  • Windows: C:\ProgramData\DockerDesktop
  • MacOS: ~/Library/Containers/com.docker.docker/Data/vms/0/

  1. Windows and MacOs OS works in a slighly different maner while running containers, In macOS and Windows, Docker runs Linux containers in a virtual environment. For MacOS, virtual image data is located in:

    ~/Library/Containers/com.docker.docker/Data/vms/0

    Within the virtual image, the path is the default Docker path /var/lib/docker.

    You can investigate your Docker root directory by creating a shell in the virtual environment:

    $ screen ~/Library/Containers/com.docker.docker/Data/vms/0/tty

  2. On Windows, Linux containers are run in a minimal Hyper-V based virtual environment. The configuration and the virtual image to execute linux images are saved in the default Docker root folder.

    C:\ProgramData\DockerDesktop

    If you inspect regular images then you will get linux paths like:

    $ docker inspect nginx

    … “UpperDir”: “/var/lib/docker/overlay2/585…9eb/diff” … You can connect to the virtual image by:

    docker run -it –privileged –pid=host debian nsenter -t 1 -m -u -i sh

  3. There, you can go to the referenced location: 
       $ cd /var/lib/docker/overlay2/585...9eb/
   $ ls -lah

   drwx------    4 root     root        4.0K Feb  6 06:56 .
   drwx------   13 root     root        4.0K Feb  6 09:17 ..
   drwxr-xr-x    3 root     root        4.0K Feb  6 06:56 diff
   -rw-r--r--    1 root     root          26 Feb  6 06:56 link
   -rw-r--r--    1 root     root          57 Feb  6 06:56 lower
   drwx------    2 root     root        4.0K Feb  6 06:56 work
  4. Inside the /var/lib/docker folder, different information are stored for example data for containers, volumes, builds, networks, and clusters. The internal stricture of the docker root directory is as below 
      $ ls -la /var/lib/docker

  total 152
  drwx--x--x   15 root     root          4096 Feb  1 13:09 .
  drwxr-xr-x   13 root     root          4096 Aug  1  2019 ..
  drwx------    2 root     root          4096 May 20  2019 builder
  drwx------    4 root     root          4096 May 20  2019 buildkit
  drwx------    3 root     root          4096 May 20  2019 containerd
  drwx------    2 root     root         12288 Feb  3 19:35 containers
  drwx------    3 root     root          4096 May 20  2019 image
  drwxr-x---    3 root     root          4096 May 20  2019 network
  drwx------    6 root     root         77824 Feb  3 19:37 overlay2
  drwx------    4 root     root          4096 May 20  2019 plugins
  drwx------    2 root     root          4096 Feb  1 13:09 runtimes
  drwx------    2 root     root          4096 May 20  2019 swarm
  drwx------    2 root     root          4096 Feb  3 19:37 tmp
  drwx------    2 root     root          4096 May 20  2019 trust
  drwx------   15 root     root         12288 Feb  3 19:35 volumes
  5. If using the default storage driver ###overlay2, then the Docker images are stored in /var/lib/docker/overlay2. There, you can find different files that represent read- only layers of a Docker image and a layer on top of it that contains your changes. 
       $ docker image pull nginx
   $ docker image inspect nginx

    [ { “Id”: “sha256:207…6e1”, “RepoTags”: [ “nginx:latest” ], “RepoDigests”: [ “nginx@sha256:ad5…c6f” ], “Parent”: “”, … “Architecture”: “amd64”, “Os”: “linux”, “Size”: 126698063, “VirtualSize”: 126698063, “GraphDriver”: { “Data”: { “LowerDir”: “/var/lib/docker/overlay2/585…9eb/diff: /var/lib/docker/overlay2/585…9eb/diff”, “MergedDir”: “/var/lib/docker/overlay2/585…9eb/merged”, “UpperDir”: “/var/lib/docker/overlay2/585…9eb/diff”, “WorkDir”: “/var/lib/docker/overlay2/585…9eb/work” }, …

The LowerDir contains the read-only layers of an image. The read-write layer that represents changes are part of the UpperDir. In my case, the NGINX UpperDir folder contains the log files:

$ ls -la /var/lib/docker/overlay2/585...9eb/diff

total 8
drwxr-xr-x    2 root     root    4096 Feb  2 08:06 .
drwxr-xr-x    3 root     root    4096 Feb  2 08:06 ..
lrwxrwxrwx    1 root     root      11 Feb  2 08:06 access.log -> /dev/stdout
lrwxrwxrwx    1 root     root      11 Feb  2 08:06 error.log -> /dev/stderr

The MergedDir represents the result of the UpperDir and LowerDir that is used by Docker to run the container. The WorkDir is an internal directory for overlay2 and should be empty.

Docker Volumes It is possible to add a persistent store to containers to keep data longer than the container exists or to share the volume with the host or with other containers. A container can be started with a volume by using the -v option:

$ docker run --name nginx_container -v /var/log nginx

We can get information about the connected volume location by:

$ docker inspect nginx_container

… “Mounts”: [ { “Type”: “volume”, “Name”: “1e4…d9c”, “Source”: “/var/lib/docker/volumes/1e4…d9c/_data”, “Destination”: “/var/log”, “Driver”: “local”, “Mode”: “”, “RW”: true, “Propagation”: “” } ], … The referenced directory contains files from the location /var/log of the NGINX container.

$ ls -lah /var/lib/docker/volumes/1e4...d9c/_data

total 88
drwxr-xr-x    4 root     root        4.0K Feb  3 21:02 .
drwxr-xr-x    3 root     root        4.0K Feb  3 21:02 ..
drwxr-xr-x    2 root     root        4.0K Feb  3 21:02 apt
-rw-rw----    1 root     43             0 Jan 30 00:00 btmp
-rw-r--r--    1 root     root       34.7K Feb  2 08:06 dpkg.log
-rw-r--r--    1 root     root        3.2K Feb  2 08:06 faillog
-rw-rw-r--    1 root     43         29.1K Feb  2 08:06 lastlog
drwxr-xr-x    2 root     root        4.0K Feb  3 21:02 nginx
-rw-rw-r--    1 root     43             0 Jan 30 00:00 w

Cleaning up space used by Docker

Most of the times the storage used by the docker can fill up quite quickly, we need to investigate it clean it up. It is recommended to use the Docker command to clean up unused containers, images. Container, networks, images, and the build cache can be cleaned up by executing:

$ docker system prune -a

Additionally, you can also remove unused volumes by executing:

$ docker volumes prune

Summary

Docker is an important part of many people’s environments and tooling. Sometimes, Docker feels a bit like magic by solving issues in a very smart way without telling the user how things are done behind the scenes. Still, Docker is a regular tool that stores its heavy parts in locations that can be opened and changed.