Monday 3 September 2018

Docker on Oracle Linux

It occurred to me that if you want to start using Docker there are plenty examples that use Ubuntu as a base platform. I read a book called Learning Docker that assumes Ubuntu for the examples, for instance. I know I am quite stubborn, a "know-it-better" person, but I want to be able to do the same on Oracle Linux.

Docker on Oracle Linux turns out not too complicated. But I ran in to a caveat that I solved and want to share.

I use Vagrant to bring up an Oracle Linux Box, based on a Vagrantfile that prepares the box.With that as a starting point, I created a script that does the complete docker installation. In the following I'll build it up for you, step by step. I'll add my project to GitHub, and provide a link to the complete script in the end.


First some initialization and a function to read the property file:
SCRIPTPATH=$(dirname $0)
# Install docker on Oracle Linux.
# @author: Martien van den Akker, Darwin-IT Professionals.
function prop {
    grep "${1}" $SCRIPTPATH/|cut -d'=' -f2

DOCKER_USER=$(prop 'docker.user')

This sets the $SCRIPTPATH property as the folder where the script resides, so I can refer to other files relative to that one.
The function prop alows me to get a property from a property file. A smart function that I got from a colleague (thanks Rob). Based on this property file called

I set the DOCKER_USER and DOCKER_GROUP properties.
The DOCKER_GROUP property is hardcoded however, but that is the standard group that is created at installation of Docker to allow other users to use Docker.

Install Docker Engine

The first actual step is to install the docker engine. Now you can go for the community edition and I've seen that there are examples that pulls the docker-ce (docker Community Engine) for you. However, one of the reasons I am stubborn to stick with Oracle Linux (as you know a RedHat derivate) is that Oracle Linux is the flavor that is used with most of my customers. And if not, it is RedHat. And then I just want to rely on the standard repositories.

To install the docker engine, I have to add the ol7_addons and the ol7_optional_latest repositories. During my OL prepare script, I already added the ol7_developer_EPEL repository. Then the docker-engine package can simply be installed by yum:

echo 1. Install Docker Engine
echo . add ol7_addons and ol7_optional_latest repos.
sudo yum-config-manager --enable ol7_addons
sudo yum-config-manager --enable ol7_optional_latest
echo . install docker-engine
sudo yum install -q -y docker-engine

Install Curl

For most docker related actions, it is convenient to have  curl installed as well:
echo 2. Install curl
sudo yum install -q -y  curl 

Add docker group to docker user

After the docker installation, we need to add the docker group to the docker user (in my case the sort-of default oracle user):
echo 3. Add  ${DOCKER_GROUP} group to ${DOCKER_USER}
sudo usermod -aG ${DOCKER_GROUP} ${DOCKER_USER}

This allows the ${DOCKER_USER} (set in the initialization phase) to use the docker command.

Check the docker install

Now let's add a check if docker works:
echo 4. Check Docker install
docker --version
sudo systemctl start docker
sudo systemctl status docker
This lists the version of the installed docker, then starts the docker service and lists the status of the docker.

Change the location of the docker containers

When creating a docker container/image (I leave the difference for now), these are saved by default in the location /var/lib/docker. The thing is that this is on the root disk of your installation. And it can grow quite big. For installations of oracle software for instance, I create an extra disk that I mount on /app. It would be better to have a /data mount point as well, but for now I stick with the /app data. So, I want to have docker place my images on the secondary disk. One solution used by Tim Hall, see here, is to create a second disk, format it with BTRFS, and mount it simply to /var/lib/docker.
I rather reconfigure docker to use another disk. This is taken from this article.

To implement this, we first need to know which storage driver Docker uses. We get this from the command docker info, as follows:
echo 5. Change docker default folder
# According to oracle-base you should create a filesystem, preferably using BTRFS, for the container-home. 
# But let's stick with ext4.
## Adapted from
echo 5.1. Find Storage Driver
GREP_STRG_DRVR=$(sudo docker info |grep "Storage Driver")
echo "Storage Driver: ${DOCKER_STORAGE_DRVR}"

This mentions me the overlay2 driver. Then we need to stop docker:
echo 5.2. Stop docker
sudo systemctl stop docker

And then create the folder where we want to store the images:
echo 5.3. Add reference to data folders for storage.
echo mkdir -p ${DOCKER_DATA_HOME}
sudo mkdir ${DOCKER_DATA_HOME}

Now I found a bit of a problem with my solution here. When I reconfigure docker to use my custom folder, it turns out that on my system the filesystem is not writable from the docker image. If you want to install software in your image, it of course wants to write the files. And this is prevented. After quite some searching, I came on this question on stackoverflow. It turns out that selinux enforces a policy that prevents writing of docker to a custom device. This can be simply circumvented by disabling the enforcing:
echo disable selinux enforcing
sudo setenforce 0

This disables, as said, the enforcing of selinux. I would say this should be a bit more nuanced. But I don't have that at hand. This however, solved my problem.
Now all is left to configure docker to use the custom folder. Docker is started using a script. In Oracle Linux this is quite conveniently setup. In the folder /etc/sysconfig you find a few config scripts, amongst others a script called: docker-storage. This is a proper to add options. When you set the DOCKER_STORAGE_OPTIONS variable, it is added to the command line. So we simply need to add the line:
DOCKER_STORAGE_OPTIONS = --graph="/app/docker/data" --storage-driver=overlay2

, to the file /etc/sysconfig/docker-storage. This can be done with the following snippet:
sudo sh -c "echo 'DOCKER_STORAGE_OPTIONS = --graph=\"${DOCKER_DATA_HOME}\" --storage-driver=${DOCKER_STORAGE_DRVR}' >> ${DOCKER_STORAGE_CFG}"

And then finish up with starting docker service again:
echo 5.4 Reload deamon
sudo systemctl daemon-reload 
echo 5.5 Start docker again
sudo systemctl start docker


Last week I was at the #PaaSSummerCamp in Lisbon. I did some labs with my docker installation, that resulted in the permission problem. As mentioned I resolved that and I could run the labs succesfully with docker containers from Oracle. So, I concluded that this script should suffice. You can download the complete script at my GitHub vagrant repo.

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