Real-world Docker Series: Installing Docker

Since this is intended to be a real-world Docker series, we're focusing on Red Hat Enterprise Linux 7.  CentOS7 will be very similar, but make sure you understand the differences between the OSes.

The first thing to do is to make sure you have the extras repository enabled.
yum repolist enabled|grep server-extras
!rhel-7-server-extras-rpms/x86_64                    Red Hat Enterprise L   112

If you don't see it listed, run:
subscription-manager repos --enable=rhel-7-server-extras-rpms

Run the first command again, to ensure the repository is now enabled.


Installing Docker:
sudo yum install docker -y

This will install docker and its dependencies, which should include the following:
docker.x86_64                    1.7.1-115.el7           @rhel-7-server-extras-rpms
docker-selinux.x86_64            1.7.1-115.el7           @rhel-7-server-extras-rpms
docker-logrotate.x86_64          1.7.1-115.el7           rhel-7-server-extras-rpms
docker-python.x86_64             1.4.0-115.el7           rhel-7-server-extras-rpms
docker-registry.noarch           0.6.8-8.el7             rhel-7-server-extras-rpms
docker-registry.x86_64           0.9.1-7.el7             rhel-7-server-extras-rpms

Next: Configuring Docker Storage

Real-world Docker Series: Tagging Images

Once a container has been loaded (or pulled) you can tag it to make it more useful to your project. A tag is very similar to a repository tag when working with a version control system. You'll want a useful tag name to be able to manage your containers easily later.

Run 'docker images'

Notice the container id listed. We'll need that to tag the container.

Run 'docker tag <image id> <your tag name>'

Verify your tag by running 'docker images' again.  It will now appear with the tag you provided.

A single docker container image can be used by many running containers. This is done by making use of the –name <desired running name> flag during a 'docker run'.  We'll go over this in detail later.

Next: Bind Mounting Persistent Storage & Ports

Real-world Docker Series: Loading Pre-Built Container Images

When provided with a pre-built container (probably from a developer) outside of a docker registry, you can use the docker load command to import the container.

Run:

 docker load -i <container package>.tar

Verify the container image was loaded:

docker images

You will see a container listed with an id only.

You can also check out hub.docker.com to 'docker pull' pre-built images.

After pulling a docker image, you will also see them listed with 'docker images'.

Next: Tagging Container Images

Real-world Docker Series: Intro

If you found your way to my little blog, then you have probably already heard of Docker, the container engine.  I've been doing a great deal of work surrounding Docker in my profession, and this is all so new, it's hard to get your finger on the pulse of where to get started, and where to go next.  This is an attempt to take you from start to finish on setting up a real-world Docker environment.

With this series of posts, I plan to focus on the administration side of things, mainly focusing on configuration and best-practices surrounding Docker.  We'll cover more than the basics found at the Docker Getting Started Page, and provide real-world examples of using containers.  For now we're focusing on just Docker, and as time passes I'll put together some posts on using Kubernetes to control and scale clustered container environments.

Next: Installing Docker

Real-world Docker Series: Configuring Docker Storage

Once the docker package and dependencies have been installed, you will want to configure storage for your containers.

Docker storage is intended to store your container's images.  When you run 'docker pull <container name>' docker will store the data in this space.  Containers are typically very small, and usually about 300-600MB.

Docker, by default, utilizes loopback storage devices. These create a virtual device at /var/lib/docker/devicemapper and use local storage. Due to performance degradation with loopback devices, the recommended method of container storage is to utilize the docker thin pool. Take a look at the contents of /etc/sysconfig/docker-storage.  This will change after configuring the docker-pool.

The docker thin pool can make use of any block device and create a thin logical volume, and can be configured to automatically grow when new space is added to its volume group (default.)  If you have worked with VMWare ESX you'll get the idea of how a thin volume works.

To configure the docker thin pool, we will use the docker-storage-setup file. First ensure you have a new block device (disk) added to the docker host. When added, get the device name by doing a 'fdisk -l' and identify the appropriate device.

DO NOT START THE DOCKER DAEMON YET

Replacing /dev/sdX with your block device found from 'fdisk -l':

sudo vi /etc/sysconfig/docker-storage-setup

Add the following:

DEVS=”/dev/sdX” #can be a comma separated list (replace /dev/sdX with the device identified with fdisk -l)

VG=vg_docker #volume group name that docker will generate for the docker-pool

Save the file.

The docker-storage-setup script will automatically generate the appropriate thin logical volumes and volume group when the docker daemon starts, or by running docker-storage-setup manually. ****NOTE: There's currently a bug with docker-storage-setup, and the block device must be 8GB or greater.

Start the docker daemon:

sudo systemctl start docker

If you'd like, you can now enable the docker service to start at boot:

sudo systemctl enable docker

Due to the docker-pool being a LVM thin volume, you will not see the volume when running 'df -h'.  To verify the volume has been configured:

Run 'lvs'

Verify there is a new Logical Volume with the name docker-pool.

Run 'vgdisplay'

Verify there is a new Volume Group with the name you specified in docker-storage-setup (vg_docker from above.)

Run 'docker info'

This will display detailed info on how the block device was broken up between metadata and data volumes, and show you the available data storage.  To learn more about thin Logical Volumes, run 'man lvmthin'.

Once you have confirmed that everything looks good, you should remove the /etc/sysconfig/docker-storage-setup file. If you don't, I've seen the docker service not start in some instances, and upon examining your logs, you will find that it is due to existing partitions on the specified block device from the /etc/sysconfig/docker-storage-setup file.

 Take a look at /etc/sysconfig/docker-storage. You will see that this was automatically configured to utilize the new docker pool.

Next: Loading Pre-built Docker Container Images