Mounting storage

The cloud-config mount unit mechanism is used to attach additional filesystems to Container Linux nodes, whether such storage is provided by an underlying cloud platform, physical disk, SAN, or NAS system. By systemd convention, mount unit names derive from the target mount point, with interior slashes replaced by dashes, and the .mount extension appended. A unit mounting onto /var/www is thus named var-www.mount.

Mount units name the source filesystem and target mount point, and optionally the filesystem type. Cloud-config writes mount unit files beneath /etc/systemd/system. Systemd mounts filesystems defined in such units at boot time. The following example mounts an EC2 ephemeral disk at the node's /media/ephemeral directory, and is therefore named media-ephemeral.mount:

#cloud-config

coreos:
  units:
    - name: media-ephemeral.mount
      command: start
      content: |
        [Mount]
        What=/dev/xvdb
        Where=/media/ephemeral
        Type=ext3
CoreOS cloud-configs can validated using the online validator.

Use attached storage for Docker

Docker containers can be very large and debugging a build process makes it easy to accumulate hundreds of containers. It's advantageous to use attached storage to expand your capacity for container images. Be aware that some cloud providers treat certain disks as ephemeral and you will lose all Docker images contained on that disk.

We're going to mount a ext4 device to /var/lib/docker, where Docker stores images. We can do this on the fly when the machines starts up with a oneshot unit that formats the drive and another one that runs afterwards to mount it. Be sure to hardcode the correct device or look for a device by label:

#cloud-config
coreos:
  units:
    - name: format-ephemeral.service
      command: start
      content: |
        [Unit]
        Description=Formats the ephemeral drive
        After=dev-xvdb.device
        Requires=dev-xvdb.device
        [Service]
        Type=oneshot
        RemainAfterExit=yes
        ExecStart=/usr/sbin/wipefs -f /dev/xvdb
        ExecStart=/usr/sbin/mkfs.ext4 -F /dev/xvdb
    - name: var-lib-docker.mount
      command: start
      content: |
        [Unit]
        Description=Mount ephemeral to /var/lib/docker
        Requires=format-ephemeral.service
        After=format-ephemeral.service
        [Mount]
        What=/dev/xvdb
        Where=/var/lib/docker
        Type=ext4
    - name: docker.service
      drop-ins:
        - name: 10-wait-docker.conf
          content: |
            [Unit]
            After=var-lib-docker.mount
            Requires=var-lib-docker.mount
CoreOS cloud-configs can validated using the online validator.

Creating and mounting a btrfs volume file

Container Linux 561.0.0 and later are installed with ext4 + overlayfs to provide a layered filesystem for the root partition. Installations from prior to this, use btrfs for this functionality. If you'd like to continue using btrfs on newer Container Linux machines, you can do so with two systemd units: one that creates and formats a btrfs volume file and another that mounts it.

In this example, we are going to mount a new 25GB btrfs volume file to /var/lib/docker, and one can verify that Docker is using the btrfs storage driver once the Docker service has started by executing sudo docker info. We recommend allocating no more than 85% of the available disk space for a btrfs filesystem as journald will also require space on the host filesystem.

#cloud-config
coreos:
  units:
    - name: format-var-lib-docker.service
      command: start
      content: |
        [Unit]
        Before=docker.service var-lib-docker.mount
        ConditionPathExists=!/var/lib/docker.btrfs
        [Service]
        Type=oneshot
        ExecStart=/usr/bin/truncate --size=25G /var/lib/docker.btrfs
        ExecStart=/usr/sbin/mkfs.btrfs /var/lib/docker.btrfs
    - name: var-lib-docker.mount
      enable: true
      content: |
        [Unit]
        Before=docker.service
        After=format-var-lib-docker.service
        Requires=format-var-lib-docker.service
        [Install]
        RequiredBy=docker.service
        [Mount]
        What=/var/lib/docker.btrfs
        Where=/var/lib/docker
        Type=btrfs
        Options=loop,discard
CoreOS cloud-configs can validated using the online validator.

Note the declaration of ConditionPathExists=!/var/lib/docker.btrfs. Without this line, systemd would reformat the btrfs filesystem every time the machine starts.

Mounting NFS exports

This cloud-config excerpt enables the NFS host monitor rpc.statd(8), then mounts an NFS export onto the Container Linux node's /var/www.

#cloud-config
coreos:
  units:
    - name: rpc-statd.service
      command: start
      enable: true
    - name: var-www.mount
      command: start
      content: |
        [Mount]
        What=nfs.example.com:/var/www
        Where=/var/www
        Type=nfs
CoreOS cloud-configs can validated using the online validator.

To declare that another service depends on this mount, name the mount unit in the dependent unit's After and Requires properties:

[Unit]
After=var-www.mount
Requires=var-www.mount

If the mount fails, dependent units will not start.

Further reading

Check the systemd mount docs to learn about the available options. Examples specific to EC2, Google Compute Engine and Rackspace Cloud can be used as a starting point.