Daniel Brunner

Mind the storage driver for Ubuntu cloud images (on Azure)

:: IT

A few days ago I wanted to build Firefox OS’ newest release for a friend. Because I did not wanted these GB of code, binaries etc. on my notebook I fired up an Ubuntu image on Microsoft Azure. I feared that at a certain point in the build process I may had to download everything to my local machine and therefore I installed Docker via a simple

sudo apt-get install docker.io

Then I started the build process as laid out on Mozilla’s Developer Network. But, during downloading the source code (that’s about 12 GB of Git repositories from Mozilla and Android), I got a “no more space left on device”. That was strange: I had a 100 GB volume attached to the VM and enough space and inodes left. After some searching I asked on the IRC channel and got a good hint: “What’s your storage driver?”

Well, I thought that it’s AUFS; I wanted to add “as usual” because AUFS was available on my notebook from the beginning. But a docker.io info gave me:

$ sudo docker.io info
Containers: 0
Images: 0
Storage Driver: devicemapper
 Pool Name: docker-8:1-131188-pool
 Data file: /var/lib/docker/devicemapper/devicemapper/data
 Metadata file: /var/lib/docker/devicemapper/devicemapper/metadata
 Data Space Used: 291.5 Mb
 Data Space Total: 102400.0 Mb
 Metadata Space Used: 0.7 Mb
 Metadata Space Total: 2048.0 Mb
Execution Driver: native-0.1
Kernel Version: 3.13.0-29-generic
WARNING: No swap limit support

I then learned that somehow the DeviceMapper driver only allows a certain amount of diffs and I reached that amount with my build process. (Maybe it’s possible to relax that restriction but I do not know how.)

I learned as well that the Ubuntu cloud image that is provided by Microsoft Azure doesn’t have AUFS support. Therefore Docker uses the DeviceMapper storage driver instead. After I installed the AUFS support I could export the images, change the storage driver and import the images again.

It would be nice seeing the Docker documentation being more detailed on those storage drivers.

(Update 2014–10–23) Thanks to this blog post from Iron.io I found some documentation of the devicemapper storage driver. It is located in the Repository.