How to install and use docker with btrfs on CentOS 7

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In this tutorial I want to show how you can install docker on CentOS 7 and using a BTRFS partition as underlying store.

Install docker

$ yum update
Geladene Plugins: fastestmirror, priorities
Loading mirror speeds from cached hostfile
 * base: centos.mirror.sharkservers.co.uk
 * extras: centosmirror.netcup.net
 * updates: mirror.softaculous.com
10055 packages excluded due to repository priority protections
No packages marked for update

If you see a line like 10055 packages excluded due to repository priority protections, then a yum configuration is needed. The message means some packages are held by more than one repository. The priorities plugin choose packages from the highest-priority repository, excluding duplicate entries from other repos. If you don’t update first, then yum install docker will not work because of dependency problems. To make this to work you have to edit /etc/yum/pluginconf.d/priorities.conf and change the content to:

[main]
enabled=0
check_obsoletes=1

Now try again to install docker:

$ yum update
$ yum install docker

Now you can enable docker to start on boot time:

$ systemctl enable docker.service
ln -s '/usr/lib/systemd/system/docker.service' '/etc/systemd/system/multi-user.target.wants/docker.service'

Let’s check the status of the service:

$ systemctl status docker.service
docker.service - Docker Application Container Engine
   Loaded: loaded (/usr/lib/systemd/system/docker.service; enabled)
   Active: inactive (dead)
     Docs: http://docs.docker.com

Docker is still not running. Now reboot your machine or start docker with:

$ systemctl start docker.service

Afterwards we want to see some docker information to confirm everything works as expected:

$ docker info
Containers: 0
Images: 0
Storage Driver: devicemapper
 Pool Name: docker-253:1-683-pool
 Pool Blocksize: 65.54 kB
 Data file: /var/lib/docker/devicemapper/devicemapper/data
 Metadata file: /var/lib/docker/devicemapper/devicemapper/metadata
 Data Space Used: 307.2 MB
 Data Space Total: 107.4 GB
 Metadata Space Used: 733.2 kB
 Metadata Space Total: 2.147 GB
 Library Version: 1.02.84-RHEL7 (2014-03-26)
Execution Driver: native-0.2
Kernel Version: 3.10.0-123.el7.x86_64
Operating System: CentOS Linux 7 (Core)

Device mapper thin provisioning

By default docker uses the device mapper thin provisioning to manage Docker containers if AUFS is not available on the operating system. This is the case if you install docker on a CentOS 7 system for example. For the default storage type “device mapper” no additional configuration is needed. The drawback all the containers are stored in the root partition under /var/lib/docker. Enter the following to see more information on a default docker system.

$ sudo lsblk
NAME                         MAJ:MIN RM  SIZE RO TYPE MOUNTPOINT
fd0                            2:0    1    4K  0 disk 
sda                            8:0    0    8G  0 disk 
├─sda1                         8:1    0  500M  0 part /boot
└─sda2                         8:2    0  7.5G  0 part 
  ├─centos-swap              253:0    0  820M  0 lvm  [SWAP]
  └─centos-root              253:1    0  6.7G  0 lvm  /
sr0                           11:0    1 1024M  0 rom  
loop0                          7:0    0  100G  0 loop 
└─docker-253:1-25857769-pool 253:2    0  100G  0 dm   
loop1                          7:1    0    2G  0 loop 
└─docker-253:1-25857769-pool 253:2    0  100G  0 dm   

Notice the loopback mounted device. You can use docker in this way on a developer machine but don’t use it on a production system. Furthermore the size is 100GB maximum. The real disk usage is less, only so much your docker containers currently need. If you need a store more than 100GB or a faster one you can use a real device instead of a file-backed loop device. More about the device mapper can be found on Jérôme Petazzoni’s blog: Resizing Docker containers with the Device Mapper plugin.

Docker and btrfs

Btrfs is a new copy on write (CoW) filesystem for Linux, you can find more inforamtion about btrfs here. In the redhat developer blog you can read: btrfs seems the most natural fit for Docker. If you install a new centos operating system make sure to create a partition with the Btrfs filesystem and mount it to /var/lib/docker. If you have installed your system already then use the following commands (make sure vda3 or another empty partition exists):

$ systemctl stop docker
$ rm -rf /var/lib/docker
$ yum install -y btrfs-progs btrfs-progs-devel
$ mkfs.btrfs -f /dev/vda3 (caution this will delete all the datas on /dev/vda3!)
$ mkdir /var/lib/docker
$ echo "/dev/vda3 /var/lib/docker btrfs defaults 0 0" >> /etc/fstab
$ mount -a

If you are not sure about your harddisk partitions you can use the following commands to show, delete or create new partitions:

$ cat /proc/partitions
major minor  #blocks  name

 253        0 1023410176 vda
 253        1    7168000 vda1
 253        2    1024000 vda2
 253        3  419430400 vda3
 253        4  595786752 vda4
  11        0    1048575 sr0
# df -h
Dateisystem    Größe Benutzt Verf. Verw% Eingehängt auf
/dev/vda1       6,7G    1,2G  5,1G   19% /
devtmpfs        7,8G       0  7,8G    0% /dev
tmpfs           7,8G       0  7,8G    0% /dev/shm
tmpfs           7,8G    8,3M  7,8G    1% /run
tmpfs           7,8G       0  7,8G    0% /sys/fs/cgroup
/dev/vda3       400G    512K  398G    1% /var/lib/docker
/dev/vda4       560G     73M  531G    1% /data
fdisk /dev/vda

Now adapt the docker configuration for usingn btrfs, it should look similar to the following afterwards:

# /etc/sysconfig/docker

# Modify these options if you want to change the way the docker daemon runs
#OPTIONS=--selinux-enabled -H fd://
OPTIONS=-H fd:// -D -s btrfs

# Location used for temporary files, such as those created by
# docker load and build operations. Default is /var/lib/docker/tmp
# Can be overriden by setting the following environment variable.
# DOCKER_TMPDIR=/var/tmp

Because btrfs does not currently support SELinux the OPTIONS line don’t have the switch –selinux-enabled anymore. -s btrfs forces the Docker runtime to use the btrfs storage driver. That’s all now start the docker daemon and check the status by typing:

$ systemctl start docker
$ systemctl status docker

Now confirm the new store type with:

$ docker info
Containers: 0
Images: 0
Storage Driver: btrfs
Execution Driver: native-0.2
Kernel Version: 3.10.0-123.13.2.el7.x86_64
Operating System: CentOS Linux 7 (Core)
Debug mode (server): true
Debug mode (client): false
Fds: 10
Goroutines: 11
EventsListeners: 0
Init SHA1: c906504aa058139c1d0569ecd0aa5f462a73440f
Init Path: /usr/libexec/docker/dockerinit

Now try to pull a docker image and run a container from it.

$ docker pull busybox
$ docker images
$ docker run -it --rm busybox

Type exit and enter, if you want stop the busybox container. More docker commands can you find here.

Now I hope you are able to use docker with btrfs. Thanks for reading my blog and drop me a mail, if you have any questions.

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