Data volumes have been available since version 1 of the
:doc:`../api/docker_remote_api`
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 persistant 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`
Each container can have zero or more data volumes.
Getting Started
...............
Using data volumes is as simple as adding a new flag: ``-v``. The parameter ``-v`` can be used more than once in order to create more volumes within the new container. The example below shows the instruction to create a container with two new volumes::
docker run -v /var/volume1 -v /var/volume2 shykes/couchdb
For a Dockerfile, the VOLUME instruction will add one or more new volumes to any container created from the image::
VOLUME ["/var/volume1", "/var/volume2"]
Create a new container using existing volumes from an existing container:
COUCH1=$(sudo docker run -d -v /var/lib/couchdb shykes/couchdb:2013-05-03)
From the container id of that previous container ``$COUCH1`` it's possible to create new container sharing the same volume using the parameter ``-volumes-from container_id``::
COUCH2=$(sudo docker run -d -volumes-from $COUCH1 shykes/couchdb:2013-05-03)
Now, the second container has the all the information from the first volume.
Create a new container which mounts a host directory into it:
-v=[]: Create a bind mount with: [host-dir]:[container-dir]:[rw|ro].
If "host-dir" is missing, then docker creates a new volume.
This is not available for a Dockerfile due the portability and sharing purpose of it. The [host-dir] volumes is something 100% host dependent and will break on any other machine.
For example::
sudo docker run -v /var/logs:/var/host_logs:ro shykes/couchdb:2013-05-03
The command above mounts the host directory ``/var/logs`` into the container with read only permissions as ``/var/host_logs``.
