:title: Share Directories via Volumes
:description: How to create and share volumes
:keywords: Examples, Usage, volume, docker, documentation, examples

.. _volume_def:

Share Directories via Volumes
=============================

A *data volume* is a specially-designated directory within one or more
containers that bypasses the :ref:`ufs_def` to provide several useful
features for persistent or shared data:

* **Data volumes can be shared and reused between containers.** This
  is the feature that makes data volumes so powerful. You can use it
  for anything from hot database upgrades to custom backup or
  replication tools. See the example below.
* **Changes to a data volume are made directly**, without the overhead
  of a copy-on-write mechanism. This is good for very large files.
* **Changes to a data volume will not be included at the next commit**
  because they are not recorded as regular filesystem changes in the
  top layer of the :ref:`ufs_def`
* **Volumes persist until no containers use them** as they are a reference
  counted resource. The container does not need to be running to share its
  volumes, but running it can help protect it against accidental removal 
  via ``docker rm``.

Each container can have zero or more data volumes.

.. versionadded:: v0.3.0

Getting Started
...............

Using data volumes is as simple as adding a ``-v`` parameter to the ``docker run`` 
command. The ``-v`` parameter can be used more than once in order to 
create more volumes within the new container. To create a new container with 
two new volumes::

  $ docker run -v /var/volume1 -v /var/volume2 busybox true

This command will create the new container with two new volumes that 
exits instantly (``true`` is pretty much the smallest, simplest program 
that you can run). Once created you can mount its volumes in any other 
container using the ``--volumes-from`` option; irrespective of whether the
container is running or not. 

Or, you can use the VOLUME instruction in a Dockerfile to add one or more new
volumes to any container created from that image::

  # BUILD-USING:        docker build -t data .
  # RUN-USING:          docker run --name DATA data 
  FROM          busybox
  VOLUME        ["/var/volume1", "/var/volume2"]
  CMD           ["/bin/true"]

Creating and mounting a Data Volume Container
---------------------------------------------

If you have some persistent data that you want to share between containers, 
or want to use from non-persistent containers, its best to create a named
Data Volume Container, and then to mount the data from it.

Create a named container with volumes to share (``/var/volume1`` and ``/var/volume2``)::

  $ docker run -v /var/volume1 -v /var/volume2 --name DATA busybox true

Then mount those data volumes into your application containers::

  $ docker run -t -i --rm --volumes-from DATA --name client1 ubuntu bash

You can use multiple ``--volumes-from`` parameters to bring together multiple 
data volumes from multiple containers. 

Interestingly, you can mount the volumes that came from the ``DATA`` container in 
yet another container via the ``client1`` middleman container::

  $ docker run -t -i --rm --volumes-from client1 --name client2 ubuntu bash

This allows you to abstract the actual data source from users of that data, 
similar to :ref:`ambassador_pattern_linking <ambassador_pattern_linking>`.

If you remove containers that mount volumes, including the initial DATA container, 
or the middleman, the volumes will not be deleted until there are no containers still
referencing those volumes. This allows you to upgrade, or effectively migrate data volumes
between containers.

Mount a Host Directory as a Container Volume:
---------------------------------------------

::

  -v=[]: Create a bind mount with: [host-dir]:[container-dir]:[rw|ro].

You must specify an absolute path for ``host-dir``.
If ``host-dir`` is missing from the command, then docker creates a new volume.
If ``host-dir`` is present but points to a non-existent directory on the host,
Docker will automatically create this directory and use it as the source of the
bind-mount.

Note that this is not available from a Dockerfile due the portability and
sharing purpose of it. The ``host-dir`` volumes are entirely host-dependent and
might not work on any other machine.

For example::

  sudo docker run -t -i -v /var/logs:/var/host_logs:ro ubuntu bash

The command above mounts the host directory ``/var/logs`` into the
container with read only permissions as ``/var/host_logs``.

.. versionadded:: v0.5.0


Note for OS/X users and remote daemon users:
--------------------------------------------
 
OS/X users run ``boot2docker`` to create a minimalist virtual machine running the docker daemon.  That 
virtual machine then launches docker commands on behalf of the OS/X command line.   The means that ``host
directories`` refer to directories in the ``boot2docker`` virtual machine, not the OS/X filesystem.

Similarly, anytime when the docker daemon is on a remote machine, the ``host directories`` always refer to directories on the daemon's machine.

Backup, restore, or migrate data volumes
----------------------------------------

You cannot back up volumes using ``docker export``, ``docker save`` and ``docker cp``
because they are external to images.
Instead you can use ``--volumes-from`` to start a new container that can access the
data-container's volume. For example::

    $ sudo docker run --rm --volumes-from DATA -v $(pwd):/backup busybox tar cvf /backup/backup.tar /data

* ``--rm`` - remove the container when it exits
* ``--volumes-from DATA`` - attach to the volumes shared by the ``DATA`` container
* ``-v $(pwd):/backup`` - bind mount the current directory into the container; to write the tar file to
* ``busybox`` - a small simpler image - good for quick maintenance
* ``tar cvf /backup/backup.tar /data`` - creates an uncompressed tar file of all the files in the ``/data`` directory

Then to restore to the same container, or another that you've made elsewhere::

    # create a new data container
    $ sudo docker run -v /data --name DATA2 busybox true
    # untar the backup files into the new container's data volume
    $ sudo docker run --rm --volumes-from DATA2 -v $(pwd):/backup busybox tar xvf /backup/backup.tar
    data/
    data/sven.txt
    # compare to the original container
    $ sudo docker run --rm --volumes-from DATA -v `pwd`:/backup busybox ls /data
    sven.txt


You can use the basic techniques above to automate backup, migration and restore
testing using your preferred tools.

Known Issues
............

* :issue:`2702`: "lxc-start: Permission denied - failed to mount"
  could indicate a permissions problem with AppArmor. Please see the
  issue for a workaround.
* :issue:`2528`:  the busybox container is used to make the resulting container as small and
  simple as possible - whenever you need to interact with the data in the volume
  you mount it into another container.
