'Assert that the creation of a volume using ``VolumeService.create``
results in the associated filesystem also being created and the size
parameters being set according to the given ``Volume`` instance.
:param get_volume: A function which accepts a started ``VolumeService``
instance and returns a ``Volume`` instance to create.'

def _creation_test(self, get_volume):

    pool = FilesystemStoragePool(FilePath(self.mktemp()))
    service = VolumeService(FilePath(self.mktemp()), pool, reactor=Clock())
    service.startService()
    volume = get_volume(service)
    created_volume = self.successResultOf(service.create(volume))
    created_fs = pool.get(volume)
    fs_path = created_fs.get_path()
    self.assertEqual((True, volume.size, volume.size), (fs_path.isdir(), created_volume.size, created_fs.size))
