[yencabulator]

S3 or radosgw are not very relevant to a discussion about block storage devices.

Ceph's RBD is "special" in the sense that the client understands the clustering, and talks to multiple servers. If you wanted that in the mix, you'd have to run a local Ceph client -- like the Ceph software stack exposing block devices from kernel does. The only way I can see vDPA being relevant to that is to avoid middlemen layers, VM -> host kernel block device abstraction -> Ceph kernelspace RBD client. But the block device abstraction is pretty thin.

The real use case for vDPA, when talking about storage, seems to be standardizing an interface the hardware can provide. And then we're back to "why not NVMe?".

(Disclaimer: ex-Ceph-employee)