====== Docker - Volumes - Persistent Storage ======

<WRAP todo>
TODO...
</WRAP>


The approach that seems to work best for production is to use a **data only container**.

The data only container is run on a barebone image and actually does nothing except exposing a data volume.

Then you can run any other container to have access to the data container volumes:

<code bash>
docker run --volumes-from data-container some-other-container command-to-execute
</code>

  * [[http://www.offermann.us/2013/12/tiny-docker-pieces-loosely-joined.html|Here]] you can get a good picture of how to arrange the different containers.
  * [[http://crosbymichael.com/advanced-docker-volumes.html|Here]] there is a good insight on how volumes work.
UPDATE:

In [[http://container42.com/2013/12/16/persistent-volumes-with-docker-container-as-volume-pattern/|this blog post]] there is a good description of the so called **container as volume pattern** which clarifies the main point of having **data only containers**.

UPDATE 2:

[[https://docs.docker.com/engine/userguide/dockervolumes/|Docker documentation has now the DEFINITIVE description of the container as volume/s pattern]].

UPDATE 3:

Updated docs with backup/restore procedure

BACKUP:

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

  * --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

RESTORE:

<code bash>
# 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
</code>

UPDATE 4

A nice [[http://container42.com/2014/11/18/data-only-container-madness/|article from the excellent Brian Goff]] explaining why it is good to use the same image for a container and a data container.

UPDATE 5

Docker 1.9.0 has new volume API!

<code bash>
docker volume create --name hello
docker run -d -v hello:/container/path/for/volume container_image my_command
</code>

this means that the data only container pattern must be abandoned in favour of the new volumes.

Actually the volume API is only a better way to achieve what was the data-container pattern.

If you create a container with a **-v volume_name:/container/fs/path** docker will automatically create a named volume for you that can:

  - Be listed through the **docker volume ls**.
  - Be identified through the **docker volume inspect volume_name**.
  - Backed up as a normal dir.
  - Backed up as before through a **<nowiki>--volumes-from</nowiki>** connection.

The new volume api adds a useful command that let you identify dangling volumes:

<code bash>
docker volume ls -f dangling=true
</code>

And then remove it through its name:

<code bash>
docker volume rm <volume name>
</code>

----

===== References =====

http://stackoverflow.com/questions/18496940/how-to-deal-with-persistent-storage-e-g-databases-in-docker?rq=1

http://txt.fliglio.com/2013/11/creating-a-mysql-docker-container/