CoreOS ships by default with a runnable qemu image which works quite nicely. One problem i encountered fairly early is the lack of disk space.

If the reason for this is a big docker graph the solution might be to move your docker graph entirely. A nice side effect is the ability to persist your graph and optimize I/O rate by using direct disk access.

Create a seperate disk for dockers graph

I’m using lvm volumes for this:

lvcreate -L 100G -n core0docker vg0

Add the disk to your startup

Add the disk to your machine startup. In my case it’s my virsh config:

<disk type='block' device='disk'>
  <driver name='qemu' type='raw' io='native'/>
  <source dev='/dev/mapper/vg0-core0docker'/>
  <target dev='vdc' bus='virtio'/>

Reboot your coreos machine to get the new device.

Systemd mount

Create a systemd mount file media-docker.mount:

# Only mount config drive block devices automatically in virtual machines
# or any host that has it explicitly enabled and not explicitly disabled.

# Support old style setup for now
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Activate the new mount service:

cp media-docker.mount /etc/systemd/system/
systemctl enable /etc/systemd/system/media-docker.mount
systemctl start media-docker

Move your docker graph

rsync -aXS /var/lib/docker/. /media/docker/dockergraph/

We’re using rsync to handle docker sparse files correctly.

Setup docker to use new graph directory

cp /usr/lib/systemd/system/docker.service /etc/systemd/system/

Add the -g option to ExecStart to change docker graph directory:

ExecStart=/usr/lib/coreos/dockerd --daemon --host=fd:// $DOCKER_OPTS $DOCKER_OPT_BIP $DOCKER_OPT_MTU $DOCKER_OPT_IPMASQ -g /media/docker/dockergraph

Restart your docker service:

systemctl daemon-reload
systemctl restart docker

Now you’re done and you can remove your old graph.

rm -rf /var/lib/docker/