Hi Stephan,

Within Openstack there are 2 kinds of storage: ephemeral and persistent.

Ephemeral storage is used on on the compute node where the instance is running. When you startup a instance on a compute node the (glance) image is downloaded to this location and the image is run from there. Anything that happens within the virtual will be saved here on the compute node. If you are creating a lot of large instances with actual data in them you might need a lot of local storage (by default the images are sparse files so it only takes as much data as is actually used in the instance). Important to know is that the data on this disk is gone when the virtual is terminated. Also, if you want to be able to (live) migrate machines between nodes you need some kind of shared storage for the ephemeral storage between all compute nodes like nfs or glusterfs. (or you could boot from Cinder volumes)

There also is persistent storage through Cinder. These are block-devices that are connected to your instance across the network. You can boot from cinder volumes so that you don't need ephemeral storage. Cinder usually connects to some highly available block device a list of available drivers, I hear Ceph is pretty nice: https://wiki.openstack.org/wiki/CinderSupportMatrix