Docker inside CT
Since OpenVZ kernel 042stab105.4 it is possible to run Docker inside containers. This article describes how.
Contents
Prerequisites
- Kernel 042stab105.4 or later version
- Kernel modules tun, veth and bridge loaded on host (not required since vzctl 4.9 as it loads it automatically)
Container creation and tuning
- Create CentOS 7 container with enough disk space:
vzctl create $veid --ostemplate centos-7-x86_64 --diskspace 20G
- Turn on bridge feature to allow docker creating bridged network:
vzctl set $veid --features bridge:on --save
- Setup Container veth-based network:
vzctl set $veid --netif_add eth0 --save
- Allow all iptables modules to be used in containers:
vzctl set $veid --netfilter full --save
- Enable tun device access for container:
vzctl set $veid --devnodes net/tun:rw --save
- Configure custom cgroups in systemd:
- systemd reads /proc/cgroups and mounts all cgroups enabled there, though it doesn't know there's a restriction that only freezer,devices and cpuacct,cpu,cpuset can be mounted in container, but not freezer, cpu etc. separately
vzctl mount $veid echo "JoinControllers=cpu,cpuacct,cpuset freezer,devices" >> /vz/root/$veid/etc/systemd/system.conf
- Start the container:
vzctl start $veid
- If you use Debian Wheezy for your CT which does not support systemd, you can run:
mount -t tmpfs tmpfs /sys/fs/cgroup mkdir /sys/fs/cgroup/freezer,devices mount -t cgroup cgroup /sys/fs/cgroup/freezer,devices -o freezer,devices mkdir /sys/fs/cgroup/cpu,cpuacct,cpuset mount -t cgroup cgroup /sys/fs/cgroup/cpu,cpuacct,cpuset/ -o cpu,cpuacct,cpuset
Prepare Docker in container
These steps are to be performed inside the container.
- Install Docker:
yum -y install docker-io
- Start docker daemon
docker -d -s vfs
Example usage
Wordpress
Use Docker to start Wordpress (official, standard way).
- Start mysql docker:
docker run --name test-mysql -e MYSQL_ROOT_PASSWORD=123 -d mysql
- Start wordpress:
docker run --name test-wordpress --link test-mysql:mysql -p 8080:80 -d wordpress
- Access wordpress server by container IP and port 8080:
http://container_ip:8080
Limitations
- Only "vfs" Docker graph driver is currently supported
- Checkpointing and live migration of a container with Docker containers inside is not supported
- Bridges cannot be created inside Docker containers running inside OpenVZ container