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I'm using Docker to deploy some services on a CentOS 6.4 server, and I'm trying to figure out how to properly backup data they generate.

For example, one of the services is a web application where users can upload files. For this container, I have a /files volume which I want to backup. Host mounts looks like they are somewhat frowned upon, because such mount is in no way portable — as said in this blog post and the docker documentation for volumes.

I know from the same blog post that I don't need a host mount to access the files in a volume, I can use docker inspect to find out where the files are.

But here's my problem: I was thinking about backing up just the dockerfiles needed to build the containers and the volumes associated with them. In the likely event that I have to restore everything from the backup, how would I go about knowing which volume directory corresponds to which container? Rebuilding the container causes the id and the volume path to change, so I would need some extra information to match them. What else, if anything, should I backup to be able to actually restore everything?

fcoelho
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2 Answers2

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You're right. Since you can have multiple containers with volumes on their own, you need to keep track which volume corresponds to which container. How to do that depends on your setup: I use the name -data for the data container, so it's obvious to which container a image belongs. That way it can be backed up like this:

VOLUME=`docker inspect $NAME-data | jq '.[0].Volumes["/path/in/container"]'`
tar -C $VOLUME . -czvf $NAME.tar.gz

Now you just need to rebuild your image and recreate your data container:

cat $NAME.tar.gz | docker run -name $NAME-data -v /path/in/container \
                              -i busybox tar -C /path/int/container -xzf -

So this means you need to backup:

  • Dockerfile
  • volume
  • volume path in container
  • name of the container the volume belongs to

Update: In the meanwhile I created a tool to backup containers and their volume(s) (container(s)): https://github.com/discordianfish/docker-backup and a backup image that can create backups and push them to s3: https://github.com/discordianfish/docker-lloyd

Johannes 'fish' Ziemke
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  • That's a fair compromise, thanks. Is there a clear advantage of using a separate container for data? – fcoelho Feb 19 '14 at 19:20
  • This again really depends on your setup. It make sense to use a data container because you can easily refer to it by using 'volumes-from' and have all the internals abstracted away: You just attach volumes from container to other containers instead of thinking in terms of path and mount points. – Johannes 'fish' Ziemke Feb 24 '14 at 11:14
  • I have this error invalid option -- z. It seems that the default tar in busybox doesn't support this. – Dzung Nguyen May 18 '14 at 15:24
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    jq is very cool, but rather than introducing a dependency, why not use `docker inspect`s built in templating like so: `VOLUME=$( docker inspect -f '{{index .Volumes "/path/in/container"}}' "${NAME}-data" )`. It's probably also wise to remind people not to expect to back up files this way while they are actively in use (e.g. databases). – mc0e Jul 29 '15 at 07:52
  • Thanks for teaching me about [jq](http://manpages.ubuntu.com/manpages/trusty/en/man1/jq.1.html)! – jpaugh Nov 03 '16 at 01:27
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    In Docker 1.8 the format has changed — `Volumes` are gone and there are `Mounts` instead with different structure. We need to do a little bit more work with `range` to find the mount point we are interested in `VOLUME=$(docker inspect --format '{{ range .Mounts }}{{ if eq .Destination "/path/in/container" }}{{ .Source }}{{ end }}{{ end }}' "${NAME}-data")` – Jarek Przygódzki Sep 08 '17 at 10:30
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In newer Docker (tested in 1.9.1, build 9894698) you can use the cp command.

Here is an example how to copy a directory from the container to the host:

docker cp wordpress:/var/www/html backups/wordpress.`date +"%Y%m%d"`/

Here is an example how to copy a directory from the container to a tar file:

docker cp wordpress:/var/www/html - > backups/wordpress.`date +"%Y%m%d"`.tar

Last but not least an example how to copy a directory from the container to a tar.gz file:

docker cp wordpress:/var/www/html - | gzip > backups/wordpress.`date +"%Y%m%d"`.tar.gz
czerasz
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    `docker cp` sends everything over the network. It's something you want to avoid especially if your Docker volume is already a btrfs volume. – Jarek Przygódzki Sep 08 '17 at 10:34
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    The question mentions backup and **restore**. A restore example in this answer using `docker cp` would be nice. – MadMike Nov 01 '17 at 10:31