openATTIC should support creating Volumes (and Shares on top of it) using the s3fs FUSE module. Instead of being backed by a block device, an S3FS volume just has a mount point, all data is transferred to an object store (e.g. Amazon S3, Ceph radosgw) in the background. All objects (files) stored via the file system can alternatively be obtained from the object store directly (e.g. via s3cmd).
This might be useful in cases where a Ceph Cluster is used for providing storage, but due to firewalling/latency issues using RBDs might not be an option.