'Running ``CreateFilesystem`` on a block device that already has a file
system fails with an exception and preserves the data.
This is because mkfs is a destructive operation that will destroy any
existing filesystem on that block device.'

def test_create_fails_on_existing_filesystem(self):

    (scenario, mountpoint) = self._make_mounted_filesystem()
    afile = mountpoint.child('file')
    afile.setContent('data')
    umount(mountpoint)
    self.failureResultOf(scenario.create(), FilesystemExists)
    self._mount(scenario, mountpoint)
    self.assertEqual(afile.getContent(), 'data')
