If you install Docker on a new Fedora or CentOS system, it is very likely that you use devicemapper. Especially in the case of Fedora cloud images, no special configuration is done to the image. While Atomic images come pre-configured with a dedicated pool. Using devicemapper with loopback can lead to unpredictable behaviour, and while OverlayFS is a nice replacement, you will not be able to use SELinux at the moment. In this short article I will show how to setup a pool for storing the Docker images.

Preparation

For this article I will be using a Fedora 24 installation on an OpenStack cloud provider*. It is a standard Cloud image, which means the root is configured as 'ext4'. I will be attaching a storage volume to the instance. Just like using Virtual Manager, the disk will be identified as /dev/vdb.

First you need to stop the Docker process and remove the existing location. This means you will loose the images, but if they are important, you can either export or push them somewhere else.

$ systemctl stop docker
$ rm -rf /var/lib/docker

After this we will create a basic LVM setup which will use the whole storage volume.

$ pvcreate /dev/vdb
$ vgcreate docker_vol /dev/vdb

We know have a volumegroup named docker_vol.

Setup Docker storage

Fedora comes with a tool that makes it easy to setup the storage for Docker, called docker-storage-setup. The configuration is done with a file, and in our case it needs to contain the identification of the volume group to use:

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

VG="docker_vol"

Now you can run:

$ docker-storage-setup

This will create the filesystem and configures Docker to use the storage pool. After this successfully finishes, it has configured the pool to use the xfs filesystem.

Verify

To verify these changes, we will start Docker and run a basic image.

$ systemctl start docker
$ docker info

Storage Driver: devicemapper
 Pool Name: docker_vol-docker--pool
 Pool Blocksize: 524.3 kB
 Base Device Size: 10.74 GB
 Backing Filesystem: xfs
 Data file: 
 Metadata file: 
 Data Space Used: 37.75 MB
 Data Space Total: 21.45 GB
 Data Space Available: 21.41 GB
 Metadata Space Used: 53.25 kB
 Metadata Space Total: 54.53 MB
 Metadata Space Available: 54.47 MB
 Udev Sync Supported: true
 Deferred Removal Enabled: true
 Deferred Deletion Enabled: true
 Deferred Deleted Device Count: 0
 Library Version: 1.02.122 (2016-04-09)

$ docker pull busybox
$ docker run -it --rm busybox
/ #

Conclusion

Configuration of storage has been really simplified and shouldn't be a reason not to do this. However, still to many people are not aware of the issues with devicemapper and loopback. Having to assign additional software to use Docker can also be a reason why people do not consider doing this, but even OverlayFS is not perfect. Using overlay with SELinux will be possible in the future, and hopefully soon these steps will also not be needed. In any case, the steps involved are simple and if you use Docker on Fedora, needed!

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