Docker inside CT

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Revision as of 16:23, 13 July 2015 by 193.16.162.1 (talk) (Container creation and tuning: debian wheezy example from Kir reply)
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Since OpenVZ kernel 042stab105.4 it is possible to run Docker inside containers. This article describes how.

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