Change the default image and container storage path of Docker on CentOS

Introduction

Docker image is a read-only template; the image can be used to create Docker containers; Docker provides a very simple mechanism to create images or update existing images, and users can even download them directly from others A ready-made image for immediate use.

1. Why?

Usually, when you start using docker, we don't care much about the default directory Docker uses to store images and containers. When experimenting more with Docker, you have to hit the panic button when Docker starts taking up a lot of space. So now it’s time to turn to troubleshooting on how to change docker’s default storage location. After going through a lot of trouble on many websites and forums, I can't figure out the steps to change the default directory for Docker images and containers on a CentOS7 host. This post provides instructions specific to Centos7 (it also applies to RHEL7). For Debian I could find a lot of documentation and "how to" articles on the internet, but not much for Centos7.

2. How?

When trying to change Docker's default storage location, we must know some important information.

The default storage location currently used by Docker

Storage driver currently used by Docker

New storage space to store images and containers

The default location of Docker is /var/lib/docker, where all current images and containers are stored. If you have any running containers, stop them and make sure no containers are running, then run the following command to determine the storage driver currently used by Docker. (You’ll find out why it’s recorded in a later article)

＃ docker info

In the output information, look for the Storage Driver line and write it down. On my host machine is devicemapper. The next step is to stop the Docker service

＃ sudo systemctl stop docker

Create a Drop-In file

The next step is to create a Drop-In file "docker.conf" in the /etc/systemd/system/docker.service.d directory. The default docker.service.d folder does not exist. So you have to create it first.

# sudo mkdir /etc/systemd/system/docker.service.d

# sudo touch /etc/systemd/system/docker.service.d/docker.conf

The reason for creating the Drop-In file is that we want the Docker service to use the specific parameters mentioned in the docker.conf file and replace the parameters used by the default service in the /lib/systemd/system/docker.service file. to cover. If you want to learn more about Drop-In, please read the system.unit documentation

Define new storage location

Now open docker.conf and add the following content:

# sudo vi /etc/systemd/system/docker.service.d/docker.conf

[Service]

ExecStart= ExecStart=/usr/bin/dockerd --graph="/mnt/new_volume" --storage-driver=devicemapper

Save and exit the VI editor, /mnt/new_volume is the new storage location, and devicemapper is the storage driver currently used by docker. If your storage driver is different, enter the value you reviewed and noted in the previous step. More information about various storage drives is available in the official Docker documentation. Now, you can reload the service daemon and start the Docker service. This will change where new images and containers are stored.

# sudo systemctl daemon-reload

# sudo systemctl start docker

To confirm that everything is OK, run the # docker info command to check the Docker root directory. It will be changed to /mnt/new_volume

What should you do if you already have existing containers and images?

If you want to migrate existing containers and images to a new location, do not reload the daemon and start the docker service after modifying docker.conf (

Note: I haven't tried the following as I don't need to keep existing containers and images, but these steps should work; please comment below if you encounter any issues and any alternative methods. I will modify the post. Before you take the plunge, back up your data.

# cp -rp /var/lib/docker /mnt/new_volume

Create soft link (

# mv /var/lib/docker /var/lib/docker-backup-2017-0510

# ln -s /mnt/new_volume/docker /var/lib/docker

Then reload the daemon and start the docker service. (

# sudo systemctl daemon-reload # sudo systemctl start docker

The existing data should now be in the source directory of the soft link, and the new containers and images will be stored in the new location, i.e. /mnt/new_volume/docker. Run # docker info to confirm.

（

...Omit output

Name: docker

ID: 5WBA:EF4D:WQ7P:DVRN:JCI4:LWDT:XSR2:G7RE:F5TI:PD3B:A57K:E4QA

Docker Root Dir: /mnt/new_volume/docker

...Omit output

Note: The fonts with italics and horizontal lines in the text mean that the author of the original text has errors (the English is not good, if there is any misunderstanding of the meaning of the original text, please point it out in time and feel free to enlighten me), the relevant positions are There are comments.

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