Small pieces

A place for small pieces I want to publish.

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

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

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 info gave me:

$ sudo 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 I found some documentation of the devicemapper storage driver. It is located in the Repository.