2014-05-21 21:05:19 +00:00
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page_title: Managing Data in Containers
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page_description: How to manage data inside your Docker containers.
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page_keywords: Examples, Usage, volume, docker, documentation, user guide, data, volumes
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# Managing Data in Containers
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2014-06-16 07:52:23 +00:00
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So far we've been introduced to some [basic Docker
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2014-05-21 21:05:19 +00:00
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concepts](/userguide/usingdocker/), seen how to work with [Docker
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images](/userguide/dockerimages/) as well as learned about [networking
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and links between containers](/userguide/dockerlinks/). In this section
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we're going to discuss how you can manage data inside and between your
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Docker containers.
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We're going to look at the two primary ways you can manage data in
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Docker.
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* Data volumes, and
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* Data volume containers.
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## Data volumes
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A *data volume* is a specially-designated directory within one or more
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containers that bypasses the [*Union File
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System*](/terms/layer/#ufs-def) to provide several useful features for
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persistent or shared data:
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- Data volumes can be shared and reused between containers
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- Changes to a data volume are made directly
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- Changes to a data volume will not be included when you update an image
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- Volumes persist until no containers use them
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### Adding a data volume
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You can add a data volume to a container using the `-v` flag with the
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`docker run` command. You can use the `-v` multiple times in a single
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`docker run` to mount multiple data volumes. Let's mount a single volume
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now in our web application container.
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$ sudo docker run -d -P --name web -v /webapp training/webapp python app.py
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This will create a new volume inside a container at `/webapp`.
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> **Note:**
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> You can also use the `VOLUME` instruction in a `Dockerfile` to add one or
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> more new volumes to any container created from that image.
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### Mount a Host Directory as a Data Volume
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In addition to creating a volume using the `-v` flag you can also mount a
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directory from your own host into a container.
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$ sudo docker run -d -P --name web -v /src/webapp:/opt/webapp training/webapp python app.py
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This will mount the local directory, `/src/webapp`, into the container as the
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`/opt/webapp` directory. This is very useful for testing, for example we can
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mount our source code inside the container and see our application at work as
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we change the source code. The directory on the host must be specified as an
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absolute path and if the directory doesn't exist Docker will automatically
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create it for you.
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2014-06-08 18:31:10 +00:00
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> **Note:**
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> This is not available from a `Dockerfile` due the portability
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> and sharing purpose of it. As the host directory is, by its nature,
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> host-dependent it might not work all hosts.
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Docker defaults to a read-write volume but we can also mount a directory
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read-only.
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$ sudo docker run -d -P --name web -v /src/webapp:/opt/webapp:ro training/webapp python app.py
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Here we've mounted the same `/src/webapp` directory but we've added the `ro`
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option to specify that the mount should be read-only.
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2014-07-04 17:52:10 +00:00
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### Mount a Host File as a Data Volume
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As well as directories, the `-v` flag can be used to mount a single file from the host into a container.
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$ sudo docker run --rm -it -v ~/.bash_history:/.bash_history ubuntu /bin/bash
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This will drop you into a bash shell in a new container, you will have your bash history from your host and when
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2014-07-05 02:46:41 +00:00
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you exit the container, the host will have the history of the commands typed while in the container.
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> **Note:**
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> Many tools used to edit files including `vi` and `sed --in-place` may result in a inode change. Since docker v1.1.0
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> this will produce a error such as "*sed: cannot rename ./sedKdJ9Dy: Device or resource busy*". In the case where you
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> want to edit the mounted file, it is often easiest to instead mount the parent directory.
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## Creating and mounting a Data Volume Container
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If you have some persistent data that you want to share between
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containers, or want to use from non-persistent containers, it's best to
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create a named Data Volume Container, and then to mount the data from
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it.
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Let's create a new named container with a volume to share.
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2014-06-24 09:24:45 +00:00
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$ sudo docker run -d -v /dbdata --name dbdata training/postgres
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You can then use the `--volumes-from` flag to mount the `/dbdata` volume in another container.
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2014-06-24 09:24:45 +00:00
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$ sudo docker run -d --volumes-from dbdata --name db1 training/postgres
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And another:
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2014-06-24 09:24:45 +00:00
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$ sudo docker run -d --volumes-from dbdata --name db2 training/postgres
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2014-06-23 15:08:34 +00:00
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You can use multiple `--volumes-from` parameters to bring together multiple data
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volumes from multiple containers.
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You can also extend the chain by mounting the volume that came from the
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`dbdata` container in yet another container via the `db1` or `db2` containers.
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2014-06-24 09:24:45 +00:00
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$ sudo docker run -d --name db3 --volumes-from db1 training/postgres
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If you remove containers that mount volumes, including the initial `dbdata`
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container, or the subsequent containers `db1` and `db2`, the volumes will not
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be deleted until there are no containers still referencing those volumes. This
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allows you to upgrade, or effectively migrate data volumes between containers.
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## Backup, restore, or migrate data volumes
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Another useful function we can perform with volumes is use them for
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backups, restores or migrations. We do this by using the
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`--volumes-from` flag to create a new container that mounts that volume,
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like so:
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$ sudo docker run --volumes-from dbdata -v $(pwd):/backup ubuntu tar cvf /backup/backup.tar /dbdata
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Here's we've launched a new container and mounted the volume from the
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`dbdata` container. We've then mounted a local host directory as
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`/backup`. Finally, we've passed a command that uses `tar` to backup the
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contents of the `dbdata` volume to a `backup.tar` file inside our
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`/backup` directory. When the command completes and the container stops
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we'll be left with a backup of our `dbdata` volume.
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You could then to restore to the same container, or another that you've made
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elsewhere. Create a new container.
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2014-06-25 05:53:59 +00:00
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$ sudo docker run -v /dbdata --name dbdata2 ubuntu /bin/bash
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Then un-tar the backup file in the new container's data volume.
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$ sudo docker run --volumes-from dbdata2 -v $(pwd):/backup busybox tar xvf /backup/backup.tar
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You can use this techniques above to automate backup, migration and
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restore testing using your preferred tools.
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# Next steps
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Now we've learned a bit more about how to use Docker we're going to see how to
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combine Docker with the services available on
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2014-06-01 20:48:04 +00:00
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[Docker Hub](https://hub.docker.com) including Automated Builds and private
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repositories.
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2014-06-01 20:48:04 +00:00
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Go to [Working with Docker Hub](/userguide/dockerrepos).
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