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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 aDockerfile
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 ro
option to specify that the mount should be read-only.
Because of limitations in the mount
function,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
andsed --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/postgres
image 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 /dbdata
then 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 dbdata
container, 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.
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