Recently Docker introduced a volume command (docker volume –help for more information) to allow users to create a logical volume and then bind-mount it inside the container at container creation/runtime. You can achieve this by:

local_volume_foobar

This will create a logical volume foobar mounted at filesystem location /var/lib/docker/volumes.

Now, you can bind mount the foobar volume into the /run directory inside the container.

bind_mount_foobar_local

While this is all good, Docker does not allow us to put a restriction on the size of the volume created. For use cases like OpenShift, where the Platform-as-a-service (PAAS) system wants to create a size-adjustable container, the docker volume command might not be good enough.

So we decided to write a docker volume plugin, which would allow us to create a size-adjustable volume and then bind-mount it inside the container.

Docker LVM Plugin

docker-lvm-plugin is a volume plugin, which creates logical volumes using LVM2. LVM2 is a userspace toolset that provides logical volume management in Linux. To support LVM2 commands used by our plugin, you would basically need devicemapper in your kernel, a userspace devicemapper support library (libdevmapper), and the userspace LVM2 tools. Most of these things should be present by default on your RHEL or Fedora linux distributions.

I would encourage you to check it out https://github.com/projectatomic/docker-lvm-plugin for more information.

The README.md has all the instructions on how to set it up and get it running. However I would still point out the important things:

  • Even though the instruction says you need to start the docker-lvm-plugin daemon using systemctl start docker-lvm-plugin, this is optional as the daemon is on-demand socket activated. What it means is, once the plugin in installed on your system, doing a docker volume ls would automatically start the daemon.

  • Since logical volumes (lv’s) are based on a volume group, it is the responsibility of the user (administrator) to provide a volume group name. You can choose an existing volume group name by listing volume groups on your system using vgs command OR create a new volume group using vgcreate command. e.g.,

    vgcreate volume_group_one /dev/hda

    where /dev/hda is your partition or whole disk on which physical volumes were created.

Update your volume group name in /etc/docker/docker-lvm-plugin.

That’s it! You are all set to use the plugin now.

Examples

To explore how this plugin will work, here are some use-case examples.

Volume Creation

lvm_volume_foobar_create

This will create a lvm volume named foobar with a size of 208 MB (0.2 GB).

Volume List

lvm_volume_list

This will list volumes created by all docker drivers, including the default driver (local).

Volume Inspect

lvm_volume_inspect

This will inspect foobar and return a JSON.

Volume Removal

lvm_volume_rm

This will remove the lvm volume foobar.

Bind Mount lvm volume Inside the Container

bind_mount_foobar_lvm

This will bind mount the logical volume foobar into the home directory of the container.

Hope you will enjoy playing around with this plugin!