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Have they disclosed what the block storage service is backed by? LeftHand P4000s?

They are using OpenStack so OpenStack Swift (same software powering Rackspace Cloud Files)

Block storage is like a SAN or Amazon's EBS; Object Storage is like a NAS or Amazon's S3 or Rackspace Cloud Files (Swift).

So they're not using Swift for block storage, unless they've built some clever stuff on top.

OpenStack does include a volume service for block storage, but it can use a number of back-ends.

No, actually it's not an opensource/public product providing the block storage.