In this section we are going to look Data Volumes and Data Volume Containers, which allows contaiers to have external storage. There are two primary ways to mannage data for containers:-

Data Volumes

From the Docker documentation (https://docs.docker.com/userguide/dockervolumes/), a data volume is a specially-designated directory within one or more containers that bypasses the Union File System to provide several useful features for persistent or shared data:

• Volumes are initialized when a container is created. If the container's base image contains data at the specified mount point, that data is copied into the new volume.
• Data volumes can be shared and reused between containers
• Changes to a data volume are made directly
• Changes to a data volume will not be included when you update an image
• Volumes persist until no containers use them

Data Volume containers

As a volume persist until no container uses it, we can use volume to share persistent data between containers. So, we can create named volume container and mount the data to other container.

Data Volumes

Let's create some volumes and inspect them.

Creating volumes

vagrant ssh labvm-1
sudo -s
docker run -idt -v /data --name centos1 centos bash
docker run -idt -v /data -v /code --name centos2 centos bash
docker ps
docker inspect -f “” centos2
docker inspect -f "" centos2

Mounting host directory inside container

For this demo, we would mount the host directory, inside the container in rw and ro mode.

vagrant ssh labvm-1
sudo -s
mkdir -p ./tmp/dataHost
date > /tmp/dataHost/date
docker run -it -v  /tmp/dataHost:/data --name centos3 centos bash
.. cd /data 
ls 
touch file1
CTRL+D
ls /tmp/dataHost
docker run -it -v  /tmp/dataHost:/data:ro --name centos4 centos bash
.. cd /data 
touch file2

Data Volume Containers

For this demo, we would first run a contaier with some volume (/data) in background. Then we would create two other contaiers and get the earlier created volume with --volume-from option inside them. Once we make changes in any of the mounted end-point, it should be reflected at other mount location as well.

vagrant ssh labvm-1
sudo -s
docker run -d -v /data --name data_container centos echo “data volume container”
docker run -itd --volumes-from data_container --name client1 centos bash
docker inspect -f “” client1    
docker inspect -f “” client1
docker exec -it client1 bash 
ls /
touch /data/file1
CTRL + D
ls /var/lib/docker/vfs/………..
docker run -it --volumes-from data_container --name client2 centos bash
ls /data
touch /data/file2
CTRL + D
ls /var/lib/docker/vfs/………..
docker exec -it client1 bash 
ls /data