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Docker plugin to manage Docker Volumes as BTRFS subvolumes

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BTRFS Volume plugin for Docker

This package provides a Docker volume plugin that creates a BTRFS subvolume for each container volume.

Please note this is not a BTRFS storage driver for Docker, but a plugin to manage volumes. It means you can use any storage driver, such as AUFS, this is an independant topic.


BTRFS is a next-generation copy-on-write filesystem with subvolume and snapshot support. A BTRFS subvolume can be seen as an independant file namespace that can live in a directory and can be mounted as a filesystem and snapshotted individually.

On the other hand, Docker volumes are commonly used to store persistent data of stateful containers, such as a MySQL/PostgreSQL database or an upload directory of a CMS. By default, Docker volumes are just local directories in the host filesystem. A number of Volume plugins already exist for various storage backends, including distributed filesystems, but small clusters often can’t afford to deploy a distributed filesystem.

We believe BTRFS subvolumes are a powerful and lightweight storage solution for Docker volumes, allowing fast and easy replication (and backup) across several nodes of a small cluster.


Make sure the directory /var/lib/buttervolume/ is living in a BTRFS filesystem. It can be a BTRFS mountpoint or a BTRFS subvolume or both.

You should also create the directories for the config and ssh on the host:

sudo mkdir /var/lib/buttervolume/{config,ssh}

Build and run

If you don’t want to be a contributor but just run the plugin, jump to the next section.

You first need to create a root filesystem for the plugin, using the provided Dockerfile:

git clone
cd buttervolume/docker

Then you can create the plugin:

docker plugin create anybox/buttervolume .

Now you can enable the plugin, which should start buttervolume in the plugin container:

docker plugin enable anybox/buttervolume

You can check it is responding by running a buttervolume command:

alias drunc="sudo docker-runc --root /run/docker/plugins/runtime-root/plugins.moby/"
alias buttervolume="drunc exec -t `drunc list|tail -n+2|awk '{print $1}'` buttervolume"
sudo buttervolume scheduled

You can also locally install and run the plugin with:

pyvenv venv
./venv/bin/python develop
sudo ./venv/bin/buttervolume run

Install and run

If the plugin is already pushed to the image repository, you can install it with:

docker plugin install anybox/buttervolume

Check it is running:

docker plugin ls

Define useful aliases:

alias drunc="sudo docker-runc --root /run/docker/plugins/runtime-root/plugins.moby/"
alias buttervolume="drunc exec -t `drunc list|tail -n+2|awk '{print $1}'` buttervolume"

And try a buttervolume command:

buttervolume scheduled

Or create a volume with the driver. Note that the name of the driver is the name of the plugin:

docker volume create -d anybox/buttervolume:latest myvolume


You must force disable it before reinstalling it (as explained in the docker documentation):

docker plugin disable -f anybox/buttervolume
docker plugin rm -f anybox/buttervolume
docker plugin install anybox/buttervolume


You can configure the following variables:

  • DRIVERNAME: the full name of the driver (with the tag)
  • VOLUMES_PATH: the path were the BTRFS volumes are located
  • SNAPSHOTS_PATH: the path were the BTRFS snapshots are located
  • TEST_REMOTE_PATH: the path during unit tests were the remote BTRFS snapshots are located
  • SCHEDULE: the path of the scheduler configuration
  • RUNPATH: the path of the docker run directory (/run/docker)
  • SOCKET: the path of the unix socket were buttervolume listen
  • TIMER: the number of seconds between two runs of the scheduler
  • DTFORMAT: the format of the datetime in the logs
  • LOGLEVEL: the Python log level (INFO, DEBUG, etc.)

The configuration can be done in this order of priority:

  1. from an environment variable prefixed with BUTTERVOLUME_ (ex: BUTTERVOLUME_TIMER=120)
  2. from the [DEFAULT] section of the /etc/buttervolume/config.ini file inside the container or /var/lib/buttervolume/config/config.ini on the host

Example of config.ini file:

TIMER = 120

If none of this is configured, the following default values are used:

  • DRIVERNAME = anybox/buttervolume:latest
  • VOLUMES_PATH = /var/lib/buttervolume/volumes/
  • SNAPSHOTS_PATH = /var/lib/buttervolume/snapshots/
  • TEST_REMOTE_PATH = /var/lib/buttervolume/received/
  • SCHEDULE = /etc/buttervolume/schedule.csv
  • RUNPATH = /run/docker
  • SOCKET = $RUNPATH/plugins/btrfs.sock # only if run manually
  • TIMER = 60
  • DTFORMAT = %Y-%m-%dT%H:%M:%S.%f


Running the plugin

The normal way to run it is as a new-style Docker Plugin as described above in the “Install and run” section, which will start it automatically. This will create a /run/docker/plugins/<uuid>/btrfs.sock file to be used by the Docker daemon. The <uuid> is the unique identifier of the runc/OCI container running it. This means you can probably run several versions of the plugin simultaneously but this is currently not recommended unless you keep in mind the volumes and snapshots are in the same place for the different versions. Otherwise you can configure a different path for the volumes and snapshots of each different versions using the config.ini file.

Then the name of the volume driver is the name of the plugin:

docker volume create -d anybox/buttervolume:latest myvolume


docker create --volume-driver=anybox/buttervolume:latest

However if you installed it locally as a Python distribution, you can also start it manually with:

sudo buttervolume run

In this case it will create a unix socket in /run/docker/plugins/btrfs.sock for use by Docker with the legacy plugin system. Then the name of the volume driver is the name of the socket file:

docker volume create -d btrfs myvolume


docker create --volume-driver=btrfs

When started, the plugin will also start its own scheduler to run periodic jobs (such as a snapshot, replication, purge or synchronization)

Creating and deleting volumes

Once the plugin is running, whenever you create a container you can specify the volume driver with docker create --volume-driver=btrfs --name <name> <image>. You can also manually create a BTRFS volume with docker volume create -d btrfs. It also works with docker-compose, by specifying the btrfs driver in the volumes section of the compose file.

When you delete the volume with docker rm -v <container> or docker volume rm <volume>, the BTRFS subvolume is deleted. If you snapshotted the volume elsewhere in the meantime, the snapshots won’t be deleted.

Managing volumes and snapshots

When buttervolume is installed, it provides a command line tool buttervolume, with the following subcommands:

run                 Run the plugin in foreground
snapshot            Snapshot a volume
snapshots           List snapshots
schedule            (un)Schedule a snapshot, replication or purge
scheduled           List scheduled actions
restore             Restore a snapshot (optionally to a different volume)
clone               Clone a volume as new volume
send                Send a snapshot to another host
sync                Synchronise a volume from a remote host volume
rm                  Delete a snapshot
purge               Purge old snapshot using a purge pattern

Create a snapshot

You can create a readonly snapshot of the volume with:

buttervolume snapshot <volume>

The volumes are currently expected to live in /var/lib/buttervolume/volumes and the snapshot will be created in /var/lib/docker/snapshots, by appending the datetime to the name of the volume, separated with @.

List the snapshots

You can list all the snapshots:

buttervolume snapshots

or just the snapshots corresponding to a volume with:

buttervolume snapshots <volume>

<volume> is the name of the volume, not the full path. It is expected to live in /var/lib/buttervolume/volumes.

Restore a snapshot

You can restore a snapshot as a volume. The current volume will first be snapshotted, deleted, then replaced with the snapshot. If you provide a volume name instead of a snapshot, the latest snapshot is restored. So no data is lost if you do something wrong. Please take care of stopping the container before restoring a snapshot:

buttervolume restore <snapshot>

<snapshot> is the name of the snapshot, not the full path. It is expected to live in /var/lib/docker/snapshots.

By default, the volume name corresponds to the volume the snapshot was created from. But you can optionally restore the snapshot to a different volume name by adding the target as the second argument:

buttervolume restore <snapshot> <volume>

Clone a volume

You can clone a volume as a new volume. The current volume will be cloned as a new volume name given as parameter. Please take care of stopping the container before clonning a volume:

buttervolume clone <volume> <new_volume>

<volume> is the name of the volume to be cloned, not the full path. It is expected to live in /var/lib/buttervolume/volumes. <new_volume> is the name of the new volume to be created as clone of previous one, not the full path. It is expected to be created in /var/lib/buttervolume/volumes.

Delete a snapshot

You can delete a snapshot with:

buttervolume rm <snapshot>

<snapshot> is the name of the snapshot, not the full path. It is expected to live in /var/lib/docker/snapshots.

Replicate a snapshot to another host

You can incrementally send snapshots to another host, so that data is replicated to several machines, allowing to quickly move a stateful docker container to another host. The first snapshot is first sent as a whole, then the next snapshots are used to only send the difference between the current one and the previous one. This allows to replicate snapshots very often without consuming a lot of bandwith or disk space:

buttervolume send <host> <snapshot>

<snapshot> is the name of the snapshot, not the full path. It is expected to live in /var/lib/docker/snapshots and is replicated to the same path on the remote host.

<host> is the hostname or IP address of the remote host. The snapshot is currently sent using BTRFS send/receive through ssh. This requires that ssh keys be present and already authorized on the target host (under /var/lib/buttervolume/ssh), and that the StrictHostKeyChecking no option be enabled in /var/lib/buttervolume/ssh/config on local host.

Please note you have to restart you docker daemons each time you change ssh configuration.

Synchronize a volume from another host volume

You can receive data from a remote volume, so in case there is a volume on the remote host with the same name, it will get new and most recent data from the distantant volume and replace in the local volume. Before running the rsync command a snapshot is made on the locale machine to manage recovery:

buttervolume sync <volume> <host1> [<host2>][...]

The intent is to synchronize a volume between multi hosts on running containers, so you should schedule that action on each nodes from all remote hosts.


As we are pulling data from multiple hosts we never remove data, consider removing scheduled actions before removing data on each hosts.


Make sure your application is able to handle such synchronisation

Purge old snapshots

You can purge old snapshot corresponding to the specified volume, using a retention pattern:

buttervolume purge <pattern> <volume>

If you’re unsure whether you retention pattern is correct, you can run the purge with the --dryrun option, to inspect what snapshots would be deleted, without deleting them:

buttervolume purge --dryrun <pattern> <volume>

<volume> is the name of the volume, not the full path. It is expected to live in /var/lib/buttervolume/volumes.

<pattern> is the snapshot retention pattern. It is a semicolon-separated list of time length specifiers with a unit. Units can be m for minutes, h for hours, d for days, w for weeks, y for years. The pattern should have at least 2 items.

Here are a few examples of retention patterns:

  • 4h:1d:2w:2y
    Keep all snapshots in the last four hours, then keep only one snapshot every four hours during the first day, then one snapshot per day during the first two weeks, then one snapshot every two weeks during the first two years, then delete everything after two years.
  • 4h:1w
    keep all snapshots during the last four hours, then one snapshot every four hours during the first week, then delete older snapshots.
  • 2h:2h
    keep all snapshots during the last two hours, then delete older snapshots.

Schedule a job

You can schedule a periodic job, such as a snapshot, a replication, a synchronization or a purge. The schedule it self is stored in /etc/buttervolume/schedule.csv.

Schedule a snapshot of a volume every 60 minutes:

buttervolume schedule snapshot 60 <volume>

Remove the same schedule by specifying a timer of 0 min:

buttervolume schedule snapshot 0 <volume>

Schedule a replication of volume foovolume to remote_host:

buttervolume schedule replicate:remote_host 3600 foovolume

Remove the same schedule:

buttervolume schedule replicate:remote_host 0 foovolume

Schedule a purge every hour of the snapshots of volume foovolume, but keep all the snapshots in the last 4 hours, then only one snapshot every 4 hours during the first week, then one snapshot every week during one year, then delete all snapshots after one year:

buttervolume schedule purge:4h:1w:1y 60 foovolume

Remove the same schedule:

buttervolume schedule purge:4h:1w:1y 0 foovolume

Using the right combination of snapshot schedule timer, purge schedule timer and purge retention pattern, you can create you own backup strategy, from the simplest ones to more elaborate ones. A common one is the following:

buttervolume schedule snapshot 1440 <volume>
buttervolume schedule purge:1d:4w:1y 1440 <volume>

It should create a snapshot every day, then purge snapshots everydays while keeping all snapshots in the last 24h, then one snapshot per day during one month, then one snapshot per month during only one year.

Schedule a syncrhonization of volume foovolume from remote_host1 abd remote_host2:

buttervolume schedule synchronize:remote_host1,remote_host2 60 foovolume

Remove the same schedule:

buttervolume schedule synchronize:remote_host1,remote_host2 0 foovolume

List scheduled jobs

You can list all the scheduled job with:

buttervolume scheduled

It will display the schedule in the same format used for adding the schedule, which is convenient to remove an existing schedule or add a similar one.


Copy-On-Write is disabled by default.

Why disabling copy-on-write? If your docker volume stores databases such as PostgreSQL or MariaDB, the copy-on-write feature may hurt performance a lot. The good news is that disabling copy-on-write does not prevent from doing snaphots, so we get the best of both world: good performances with the ability to do snapshots.


If your volumes directory is a BTRFS partition or volume, tests can be run with:

sudo SSH_PORT=22 python3 test

22 being the port of your running ssh server with authorized key, or using and testing the docker image (with python >= 3.5):

docker build -t anybox/buttervolume docker/
sudo docker run -it --rm --privileged \
  -v /var/lib/docker:/var/lib/docker \
  -v "$PWD":/usr/src/buttervolume \
  -w /usr/src/buttervolume \
  anybox/buttervolume test

If you have no BTRFS partitions or volumes you can setup a virtual partition in a file as follows (tested on Debian 8):

Setup BTRFS virtual partition:

sudo qemu-img create /var/lib/docker/btrfs.img 10G
sudo mkfs.btrfs /var/lib/docker/btrfs.img


you can ignore the error, in fact the new FS is formatted

Mount the partition somewhere temporarily to create 3 new BTRFS subvolumes:

sudo -s
mkdir /tmp/btrfs_mount_point
mount -o loop /var/lib/docker/btrfs.img /tmp/btrfs_mount_point/
btrfs subvolume create /tmp/btrfs_mount_point/snapshots
btrfs subvolume create /tmp/btrfs_mount_point/volumes
btrfs subvolume create /tmp/btrfs_mount_point/received
umount /tmp/btrfs_mount_point/
rm -r /tmp/btrfs_mount_point/

Stop docker, create required mount point and restart docker:

systemctl stop docker
mkdir -p /var/lib/buttervolume/volumes
mkdir -p /var/lib/docker/snapshots
mkdir -p /var/lib/docker/received
mount -o loop,subvol=volumes /var/lib/docker/btrfs.img /var/lib/buttervolume/volumes
mount -o loop,subvol=snapshots /var/lib/docker/btrfs.img /var/lib/buttervolume/snapshots
mount -o loop,subvol=received /var/lib/docker/btrfs.img /var/lib/buttervolume/received
systemctl start docker

Once you are done with your test, you can unmount those volumes and you will find back your previous docker volumes:

systemctl stop docker
umount /var/lib/buttervolume/volumes
umount /var/lib/docker/snapshots
umount /var/lib/docker/received
systemctl start docker
rm /var/lib/docker/btrfs.img

Migrate to version 3

If you’re currently using Buttervolume 1.x or 2.0 in production, you must carefully follow the guidelines below to migrate to version 3.

First copy the ssh and config files and disable the scheduler:

sudo -s
docker cp buttervolume_plugin_1:/etc/buttervolume /var/lib/buttervolume/config
docker cp buttervolume_plugin_1:/root/.ssh /var/lib/buttervolume/ssh
mv /var/lib/buttervolume/config/schedule.csv /var/lib/buttervolume/config/schedule.csv.disabled

Then stop all your containers, excepted buttervolume

Now snapshot and delete all your volumes:

volumes=$(docker volume ls -f driver=btrfs --format "{{.Name}}")
# or: # volumes=$(docker volume ls -f driver=btrfs|tail -n+2|awk '{print $2}')
echo $volumes
for v in $volumes; do docker exec buttervolume_plugin_1 buttervolume snapshot $v; done
for v in $volumes; do docker volume rm $v; done

Then stop the buttervolume container, remove the old btrfs.sock file, and restart docker:

docker stop buttervolume_plugin_1
docker rm -v buttervolume_plugin_1
rm /run/docker/plugins/btrfs.sock
systemctl stop docker

If you were using Buttervolume 1.x, you must move your snapshots to the new location:

mkdir /var/lib/buttervolume/snapshots
cd /var/lib/docker/snapshots
for i in *; do btrfs subvolume snapshot -r $i /var/lib/buttervolume/snapshots/$i; done

Restore /var/lib/docker/volumes as the original folder:

cd /var/lib/docker
mv volumes/*
umount volumes  # if this was a mounted btrfs subvolume
mv* volumes/
systemctl start docker

Change your volume configurations (in your compose files) to use the new anybox/buttervolume:latest driver name instead of btrfs

Then start the new buttervolume 3.x as a managed plugin and check it is started:

docker plugin install anybox/buttervolume:latest
docker plugin ls

Then recreate all your volumes with the new driver and restore them from the snapshots:

for v in $volumes; do docker volume create -d anybox/buttervolume:latest $v; done
alias drunc="sudo docker-runc --root /run/docker/plugins/runtime-root/plugins.moby/"
alias buttervolume="drunc exec -t `drunc list|tail -n+2|awk '{print $1}'` buttervolume"
# WARNING : check the the volume you will restore are the correct ones
for v in $volumes; do buttervolume restore $v; done

Then restart your containers, check they are ok with the correct data.

Reenable the schedule:

mv /var/lib/buttervolume/config/schedule.csv.disabled /var/lib/buttervolume/config/schedule.csv


  • Christophe Combelles
  • Pierre Verkest
  • Marcelo Ochoa


3.7 (2018-12-13)

  • unpinned urllib3

3.6 (2018-12-11)

  • fixed zombie sshd processes inside the plugin
  • minor documentation change

3.5 (2018-06-07)

  • improved documentation

3.4 (2018-04-27)

  • fix rights at startup so that ssh works

3.3 (2018-04-27)

  • Fixed a bug preventing a start in certain conditions

3.2 (2018-04-27)

  • Fixed the socket path for startup

3.1 (2018-04-27)

  • Fixed a declaration in Python 3.6
  • Automatically detects the btrfs.sock path
  • Made the runpath and drivername configurable

3.0 (2018-04-24)

  • Now use the docker managed plugin system
  • Stop the scheduler before shutdown to avoid a 5s timeout
  • Improved logging
  • Improved the migration doc from version 1 or 2

2.0 (2018-03-24)

  • BREAKING CHANGE: Please read the migration path from version 1 to version 2:
    BTRFS volumes and snapshots are now stored by default in different directories under /var/lib/buttervolume
  • Configuration possible through environment variables or a config.ini file
  • implemented VolumeDriver.Capabilities and just return 'local'
  • other minor fixes and improvements

1.4 (2018-02-01)

  • Add clone command
  • replace sync by btrfs filesystem sync

1.3.1 (2017-10-22)

  • fixed packaging (missing README)

1.3 (2017-07-30)

  • fixed the cli for the restore command

1.2 (2017-07-16)

  • fixed the purge algorithm

1.1 (2017-07-13)

  • allow to restore a snapshot to a different volume name

1.0 (2017-05-24)

  • initial release, used in production

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