def METHOD_NAME(self):
    self.test_prealloc()
    result = self.vm.qmp('blockdev-snapshot-sync', node_name='disk',
                         snapshot_file=overlay,
                         snapshot_node_name='overlay')
    self.assert_qmp(result, 'return', {})
    # on reopen to  r-o base preallocation should be dropped
    self.check_small()
    self.vm.hmp_qemu_io('drive0', 'write 1M 1M')
    result = self.vm.qmp('block-commit', device='overlay')
    self.assert_qmp(result, 'return', {})
    self.complete_and_wait()
    # commit of new megabyte should trigger preallocation
    self.check_big()