Hi, the volume you have created has been attached after the instance was created. If you look in your attached devices, you will find it as /dev/vdb.
It seems you created the instance without volume storage initially; This worked due to a bug in nova/glance, where you are actually able to deploy a volume with a "fake" size of 0 on local storage, even if local storage is not really supported.
The correct way to create your instance is to use the "Boot from image (creates a new volume)" feature in Horizon.
Here is a screenshot of how you should do it. Note that I have checked the "Delete on Terminate" option, wich will delete the volume when you delete the instance. Not sure if this is what you want.
It is ofcourse also possible to do this using Openstack Client from the command line, the workflow should be something like this:
$ openstack volume create --size 20 --description "Root volume for <SERVER-NAME>" --image <IMAGE-ID> <SERVER-NAME>-root
$ openstack server create --image <IMAGE-ID> --security-group <SECURITY-GROUP> --flavor t2.micro --key-name <KEY-NAME> --block-device-mapping vda=<ID TO VOLUME ABOVE>::1 <SERVER-NAME>
The IMAGE-ID above could be "e12ff7e3-9638-4b27-b050-616880d832af" for Ubuntu 14.04. The important thing is that it should be one of the images that are tagged as "Volume Storage", wich basically means that it is a RAW image instead of QCOW2. Please note that the need for "--image" in the "server create" stanza is only there due to a bug in OSC, but last we checked it needed to be there.
The important part is the "block-device-mapping" argument, wich actually maps the volume as the root device.
To fix your existing server, I think the following process should work:
Create a instance snapshot, so that the current volume gets uploaded into glance.
Delete your existing server.
Create a new server, with volume storage, with the image you just created as the source.
I am not entirely sure, but I think cloud-init will then resize your image at boot time.
Ofcourse, if you haven't done any major configuration changes, there is not much point of going thru this process, better to just spawn a new instance.