Docker – Move docker data (images and containers) to separate drive with different FS

docker

I am planning to move docker data (images and containers) to separate drive as they require many space.

As I know I can move /var/lib/docker to separate drive and create soft link to it. But I have read that BTRFS is a best file system option for docker.

But I suspect it is not enough to format external drive to BTRFS and move existing folder to it. As I see the content of this folder depends on the using file system.

How can I move (recreate) docker folder (images and containers) on the external drive with different file system correctly?

Best Answer

As you stated simply moving or copying the docker images/containers from the default AUFS to BTRFS formatted drive is not going to work. So as a first step, you can commit all your containers as images and use docker save commands to save the images as .tar files and then once you have prepped docker to use btrfs, you run docker load to load the images into the new storage driver.

The steps would be roughly as follows - this is what i did in my Ubuntu 14.10 box -

Save your images
docker save IMAGEID > /tmp/redis.tar
Prep btrfs

 mkfs.btrfs /dev/sdb
 mkdir /var/lib/docker-btrfs
 mount /dev/sdb /var/lib/docker-btrfs/

Stop docker and set docker to utilize btrfs and run it.

service docker stop
docker -d -s btrfs  --graph="/var/lib/docker-btrfs"  -H unix:///var/run/docker.sock

The above step can also be accomplished by modifying /etc/default/docker.

Load the images.

docker load --input /tmp/redis.tar

You should be able to see the images and run containers from here. For containers, you can also try export and import methods, i have never tried that one though.

First let's recall a few definitions

Docker images

Docker images are essentially a union filesystem + metadata. You can inspect the content of docker image union filesystem with the docker export command, and you can inspect a docker image metadata with the docker inspect command.

Data volumes

from the Docker user guide:

A data volume is a specially-designated directory within one or more containers that bypasses the Union File System to provide several useful features for persistent or shared data.

It is important to note here that a given volume (as the directory or file that contains data) is reusable only if it exists at least one docker container using it. Docker images don't have volumes, they only have metadata which eventually tells where volumes would be mounted on the union filesystem. Data volumes aren't either part of docker containers union filesystem, so where are they? under /var/lib/docker/volumes on the docker host (while containers are stored under /var/lib/docker/containers).

Data volume containers

That special type of container has nothing special. They are just stopped containers using a data volume with the sole and unique goal of having at least one container using that data volume. Remember, as soon as the last container (running or stopped) using a given data volume is deleted, that volume will become unreachable through the docker run --volumes-from option.

Working with data volume containers

How to create a data volume container

The image used to create a data volume container has no importance as such a container can remain stopped and still fill its purpose. So to create a data container named datatest_data for a volume in /datafolder you only need to run:

docker run --name datatest_data --volume /datafolder busybox true

Here base is the image name (a conveniently small one) and true is a command we provide just to avoid seeing the docker daemon complain about a missing command. Anyway after you have a stopped container named datatest_data with the sole purpose of allowing you to reach that volume with the --volumes-from option of the docker run command.

How to read from a data volume container

I know two ways of reading a data volume: the first one is through a container. If you cannot have a shell into an existing container to access that data volume, you can run a new container with the --volumes-from option for the sole purpose of reading that data.

For instance:

docker run --rm --volumes-from datatest_data busybox cat /datafolder/data.txt

The other way is to copy the volume from the /var/lib/docker/volumes folder. You can discover the name of the volume in that folder by inspecting the metadata of one of the container using the volume. See this answer for details.

Working with volumes (since Docker 1.9.0)

How to create a volume (since Docker 1.9.0)

Docker 1.9.0 introduced a new command docker volume which allows to create volumes :

docker volume create --name hello

How to read from a volume (since Docker 1.9.0)

Let say you created a volume named hello with docker volume create --name hello, you can mount it in a container with the -v option :

docker run -v hello:/data busybox ls /data

About committing & pushing containers

It should now be clear that since data volumes aren't part of a container (the union filesystem), committing a container to produce a new docker image won't persist any data that would be in a data volume.

Making backups of data volumes

The docker user guide has a nice article about making backups of data volumes.

Good article reagarding volumes: http://container42.com/2014/11/03/docker-indepth-volumes/