Data Volumes
If we use Data Volumes to persist data, we have to define a volume. Then any files or data put in that volume will outlive the container. If we remove the container, it will not remove the data volume, instead we have to take one more step to remove the volume, docker volume prune. This extra step is just for an insurance, to ensure the the data volumes are much more important than the container itself.
Let's pull the mysql image,
docker pull mysql
Now, inspect the image,
docker image inspect mysql
Inside the Config.Volumes, we should see a volumes, /var/lib/mysql.
We now run a container from the image,
docker container run -d --name mysql -e MYSQL_ALLOW_EMPTY_PASSWORD=true mysql
We can verify the container is running by,
docker container ls
In the output list, we should see the mysql container is running.
We can inspect the container,
docker container inspect mysql
Under Mounts, we should see /var/lib/mysql as mounted volumes list. When a container is running, the container assume the directory path is /var/lib/mysql, but it's actual path is defined in the Source property and in my case, the actual path is /var/lib/docker/volumes/0097b12ff7e460e44174a208e8c52ba40eeb882723696e8057906403c3a17e13/_data.
We can see the list of mounted volumes,
docker volume ls
This should give all the mounted volumes by the docker containers.
We can get the volume name by the docker container inspect mysql. From this command output, under Mounts, we can get the mounted volumes name. To inspect our specific volume, we can run inspect command by the name,
docker volume inspect 0097b12ff7e460e44174a208e8c52ba40eeb882723696e8057906403c3a17e13
In output, there is a property Mountpoint and from the linux machine, we can directly access that mount point,
cd /var/lib/docker/volumes/0097b12ff7e460e44174a208e8c52ba40eeb882723696e8057906403c3a17e13/_data
This should take us the _data directory.
Lets, remove the mysql container,
docker container rm mysql
Now, if we look for the volumes,
docker volume ls
We will notice, even though our container is removed, our data is persisted. This solves our data persistency problem.
Although, from container, we can find the mounted volumes, but from volume perspective, we can not say which container it is connected to.
Named Volumes (An Enhance of Data Volumes)
To make Data Volumes more user friendly, we can use Named Volumes. To use Named Volumes we have to specify the volume config on the docker run command with -v flag.
We can create a mysql container with Named Volume by,
docker container run -d --name mysql -e MYSQL_ALLOW_EMPTY_PASSWORD=true -v mysql-db:/var/lib/mysql mysql
Now we can easily inspect the volume by name,
docker volume inspect mysql-db
This will give the details of the mysql-db platform.
It is also possible to create volume ahead of time.