GlusterFS or Ceph RBD for storing Virtual Machine image

Not an answer you are looking for.

Did you consider local storage + regular backups for databases?

I went this route of running databases over NFS storage and ZVOLs but they were dog slow and the added complexity of going over the wire always led to problems where you dont want them.

I then switched to local RAIDed SSDs for those database servers, and they run ever since like a champ with much better performance and no storage related issues whatsoever. Although this sounds rosy, if the local disks fail (which they eventually do), they need to be restored from backups - so make sure to do backups, watch whether they happen regularly, and check whether they are restorable.

Your database size may factor into this consideration as well.