Manage Data in Containers

Manage data in containers

So far we’ve been introduced to some basic Docker concepts,seen how to work with Docker images as well as learned aboutnetworking and links between containers. In this section we’regoing to discuss how you can manage data inside and between your Dockercontainers.

We’re going to look at the two primary ways you can manage data inDocker.

• Data volumes, and
• Data volume containers.

Data volumes

A data volume is a specially-designated directory within one or morecontainers that bypasses the Union File System. Data volumes provide several useful features for persistent or shared data:

• Volumes are initialized when a container is created. If the container’sbase image contains data at the specified mount point, that existing data iscopied into the new volume upon volume initialization.
• Data volumes can be shared and reused among containers.
• Changes to a data volume are made directly.
• Changes to a data volume will not be included when you update an image.
• Data volumes persist even if the container itself is deleted.

Data volumes are designed to persist data, independent of the container’s lifecycle. Docker therefore never automatically delete volumes when you removea container, nor will it “garbage collect” volumes that are no longerreferenced by a container.

Adding a data volume

You can add a data volume to a container using the -v flag with thedocker create and docker run command. You can use the -v multiple timesto mount multiple data volumes. Let’s mount a single volume now in our webapplication container.

$ docker run -d -P --name web -v /webapp training/webapp python app.py

This will create a new volume inside a container at /webapp.

Note:You can also use the VOLUME instruction in a Dockerfile to add one ormore new volumes to any container created from that image.

Docker volumes default to mount in read-write mode, but you can also set it to be mounted read-only.

$ docker run -d -P --name web -v /opt/webapp:ro training/webapp python app.py

Locating a volume

You can locate the volume on the host by utilizing the ‘docker inspect’ command.

$ docker inspect web

The output will provide details on the container configurations including thevolumes. The output should look something similar to the following:

...
Mounts": [{"Name": "fac362...80535","Source": "/var/lib/docker/volumes/fac362...80535/_data","Destination": "/webapp","Driver": "local","Mode": "","RW": true} ] ... 

You will notice in the above ‘Source’ is specifying the location on the host and‘Destination’ is specifying the volume location inside the container. RW showsif the volume is read/write.

Mount a host directory as a data volume

In addition to creating a volume using the -v flag you can also mount adirectory from your Docker daemon’s host into a container.

$ docker run -d -P --name web -v /src/webapp:/opt/webapp training/webapp python app.py

This command mounts the host directory, /src/webapp, into the container at/opt/webapp. If the path /opt/webapp already exists inside the container’simage, the /src/webapp mount overlays but does not remove the pre-existingcontent. Once the mount is removed, the content is accessible again. This isconsistent with the expected behavior of the mount command.

The container-dir must always be an absolute path such as /src/docs.The host-dir can either be an absolute path or a name value. If yousupply an absolute path for the host-dir, Docker bind-mounts to the pathyou specify. If you supply a name, Docker creates a named volume by that name.

A name value must start with start with an alphanumeric character,followed by a-z0-9, _ (underscore), . (period) or - (hyphen).An absolute path starts with a / (forward slash).

For example, you can specify either /foo or foo for a host-dir value.If you supply the /foo value, Docker creates a bind-mount. If you supplythe foo specification, Docker creates a named volume.

If you are using Docker Machine on Mac or Windows, your Docker daemon has only limited access to your OS X or Windows filesystem. Docker Machine triesto auto-share your /Users (OS X) or C:\Users (Windows) directory. So,you can mount files or directories on OS X using.

docker run -v /Users/<path>:/<container path> ...

On Windows, mount directories using:

docker run -v /c/Users/<path>:/<container path> ...`

All other paths come from your virtual machine’s filesystem. For example, ifyou are using VirtualBox some other folder available for sharing, you need to doadditional work. In the case of VirtualBox you need to make the host folderavailable as a shared folder in VirtualBox. Then, you can mount it using theDocker -v flag.

Mounting a host directory can be useful for testing. For example, you can mountsource code inside a container. Then, change the source code and see its effecton the application in real time. The directory on the host must be specified asan absolute path and if the directory doesn’t exist Docker will automaticallycreate it for you. This auto-creation of the host path has been deprecated.

Docker volumes default to mount in read-write mode, but you can also set it tobe mounted read-only.

$ docker run -d -P --name web -v /src/webapp:/opt/webapp:ro training/webapp python app.py

Here we’ve mounted the same /src/webapp directory but we’ve added the rooption to specify that the mount should be read-only.

Because of limitations in the mountfunction,moving subdirectories within the host’s source directory can giveaccess from the container to the host’s file system. This requires a malicioususer with access to host and its mounted directory.

Note: The host directory is, by its nature, host-dependent. For thisreason, you can’t mount a host directory from Dockerfile because built imagesshould be portable. A host directory wouldn’t be available on all potentialhosts.

Volume labels

Labeling systems like SELinux require that proper labels are placed on volumecontent mounted into a container. Without a label, the security system mightprevent the processes running inside the container from using the content. Bydefault, Docker does not change the labels set by the OS.

To change a label in the container context, you can add either of two suffixes:z or :Z to the volume mount. These suffixes tell Docker to relabel fileobjects on the shared volumes. The z option tells Docker that two containersshare the volume content. As a result, Docker labels the content with a sharedcontent label. Shared volume labels allow all containers to read/write content.The Z option tells Docker to label the content with a private unshared label.Only the current container can use a private volume.

Mount a host file as a data volume

The -v flag can also be used to mount a single file - instead of justdirectories - from the host machine.

$ docker run --rm -it -v ~/.bash_history:/.bash_history ubuntu /bin/bash

This will drop you into a bash shell in a new container, you will have your bashhistory from the host and when you exit the container, the host will have thehistory of the commands typed while in the container.

Note:Many tools used to edit files including vi and sed --in-place may resultin an inode change. Since Docker v1.1.0, this will produce an error such as“sed: cannot rename ./sedKdJ9Dy: Device or resource busy”. In the case whereyou want to edit the mounted file, it is often easiest to instead mount theparent directory.

Creating and mounting a data volume container

If you have some persistent data that you want to share betweencontainers, or want to use from non-persistent containers, it’s best tocreate a named Data Volume Container, and then to mount the data fromit.

Let’s create a new named container with a volume to share.While this container doesn’t run an application, it reuses the training/postgresimage so that all containers are using layers in common, saving disk space.

$ docker create -v /dbdata --name dbdata training/postgres /bin/true

You can then use the --volumes-from flag to mount the /dbdata volume in another container.

$ docker run -d --volumes-from dbdata --name db1 training/postgres

And another:

$ docker run -d --volumes-from dbdata --name db2 training/postgres

In this case, if the postgres image contained a directory called /dbdatathen mounting the volumes from the dbdata container hides the/dbdata files from the postgres image. The result is only the filesfrom the dbdata container are visible.

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

You can also extend the chain by mounting the volume that came from thedbdata container in yet another container via the db1 or db2 containers.

$ docker run -d --name db3 --volumes-from db1 training/postgres

If you remove containers that mount volumes, including the initial dbdatacontainer, or the subsequent containers db1 and db2, the volumes will notbe deleted. To delete the volume from disk, you must explicitly calldocker rm -v against the last container with a reference to the volume. Thisallows you to upgrade, or effectively migrate data volumes between containers.

Note: Docker will not warn you when removing a container withoutproviding the -v option to delete its volumes. If you remove containerswithout using the -v option, you may end up with “dangling” volumes;volumes that are no longer referenced by a container.Dangling volumes are difficult to get rid of and can take up a large amountof disk space. We’re working on improving volume management and you can checkprogress on this in pull request #14214

Backup, restore, or migrate data volumes

Another useful function we can perform with volumes is use them forbackups, restores or migrations. We do this by using the--volumes-from flag to create a new container that mounts that volume,like so:

$ docker run --volumes-from dbdata -v $(pwd):/backup ubuntu tar cvf /backup/backup.tar /dbdata

Here we’ve launched a new container and mounted the volume from thedbdata container. We’ve then mounted a local host directory as/backup. Finally, we’ve passed a command that uses tar to backup thecontents of the dbdata volume to a backup.tar file inside our/backup directory. When the command completes and the container stopswe’ll be left with a backup of our dbdata volume.

You could then restore it to the same container, or another that you’ve madeelsewhere. Create a new container.

$ docker run -v /dbdata --name dbdata2 ubuntu /bin/bash

Then un-tar the backup file in the new container’s data volume.

$ docker run --volumes-from dbstore2 -v $(pwd):/backup ubuntu bash -c "cd /dbdata && tar xvf /backup/backup.tar"

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

Important tips on using shared volumes

Multiple containers can also share one or more data volumes. However, multiple containers writing to a single shared volume can cause data corruption. Make sure you’re applications are designed to write to shared data stores.

Data volumes are directly accessible from the Docker host. This means you can read and write to them with normal Linux tools. In most cases you should not do this as it can cause data corruption if your containers and applications are unaware of your direct access.

Next steps

Now we’ve learned a bit more about how to use Docker we’re going to see how tocombine Docker with the services available onDocker Hub including Automated Builds and privaterepositories.

Go to Working with Docker Hub.