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Change Docker root dir using systemd

The Docker root dir is usually something like /var/lib/docker by default. Here’s how to change it using a systemd .service file.

Find your current root directory using docker info.

$ docker info
    Root Dir: /var/lib/docker/aufs

Since we’re using systemd modifying the DOCKER-OPTS tag within /etc/default/docker to include -g /new/root/dir isn’t going to work. There are two options, both require you to edit your docker.service file.

Pro tip: systemctl status docker.service will print the location of this file at the top of the output

Option 1 - Direct edit to docker.service

• Edit ExecStart line to look like this ExecStart =/usr/bin/dockerd -g /new/docker/root/dir -H fd://
• systemctl daemon-reload
• systemctl restart docker
• docker info - verify the root dir has updated

Option 2 - Create a systemd drop-in service file (better way)

This option is preferred as directly editing .service files should be avoided. They may be overwritten during an update for example.

• vi /etc/systemd/system/docker.service.d/docker.root.conf and populate with:

[Service]
ExecStart=
ExecStart=/usr/bin/dockerd -g /new/docker/root -H fd://

• systemctl daemon-reload
• systemctl restart docker
• docker info - verify the root dir has updated

Option 3 - Create/Modify a json config file (even better way)

This option is preferred over Option 2 because it only changes the docker root directory and nothing else. Open or create /etc/docker/daemon.json and populate it with:

{
    "data-root": "/new/docker/root"
}

• systemctl daemon-reload
• systemctl restart docker
• docker info - verify the root dir has updated

Note - Existing Containers and Images
If you already have containers or images in /var/lib/docker you may wish to stop and back these up before moving them to the new root location. Moving can be done by either rsync -a /var/lib/docker/* /path/to/new/root or if permissions do not matter, you can simply use mv or cp too.

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