1.5. Working with volumes

🤔 I have a container with a database server running. What happens to my data when I remove the container?

It’s gone. The docker instance has no persistence layer to store data permanently, let us address that problem in this chapter.

Mounting a volume in a container

The MariaDB container is a good example for using a persistent volume. We’ll create a Docker-managed volume for persistent MariaDB data:

docker volume create volume-mariadb

Inspect your volume with:

docker volume inspect volume-mariadb

Now we create a new container named mariadb-container-with-external-volume and we mount the volume in it using the -v parameter.

docker run --name mariadb-container-with-external-volume -v volume-mariadb:/var/lib/mysql -e MARIADB_ROOT_PASSWORD=my-secret-pw -d mariadb

We test our newly gained permanent storage by adding a user to the database. First, let’t connect to the database or our new container instance:

docker exec -it mariadb-container-with-external-volume mariadb -uroot -pmy-secret-pw

Inside MariaDB run a sql statement adding the user:

use mysql;
CREATE USER 'peter'@'%' IDENTIFIED BY 'venkman';
GRANT SELECT ON mysql.user TO 'peter'@'%';

Now quit MariaDB and the container:

exit

By using volumes, we have persisted the data in our database!

We can verify this by stopping our current container and starting a new instance.

Stop and remove the mariadb-container-with-external-volume container:

docker stop mariadb-container-with-external-volume

and then

docker rm mariadb-container-with-external-volume

Next, check if the data is still available. Create a new MariaDB container with the previous volume:

docker run --name mariadb-container-with-existing-external-volume \
            -v volume-mariadb:/var/lib/mysql \
            -e MARIADB_ROOT_PASSWORD=my-secret-pw \
            -d mariadb

The moment of truth… Connect to the database server using Peter’s credentials:

docker exec -it mariadb-container-with-existing-external-volume mariadb -upeter -pvenkman

You should now be connected to your database instance as peter. You can test this by listing the users with the sql client:

SELECT User FROM mysql.user;

You should see peter in the list. You can exit the container now:

exit

Docker storage driver

When running a lot of Docker containers on a machine you usually need a lot of storage. Docker volumes and container storage are provided on a filesystem. The following link provides additional information on how to choose the correct storage setup:

https://access.redhat.com/documentation/en-us/red_hat_enterprise_linux_atomic_host/7/html/managing_containers/managing_storage_with_docker_formatted_containers

At the moment, overlay2 is the recommended storage driver .

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