Thats very interesting. When you post about that could you go into why you choose that over say large files (say 32 gb or so) that contain each of the collection of blocks? XFS or Ext4 can preallocate large chunks of the files beforehand, so the data looks nice on disk. A fully custom block level device sounds like a nice way to minimize overhead, guess it goes to show that extreme scale makes certain decisions that seem crazy from a support point of view start to make sense.