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 dockerwill 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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