| 1. Start some large update, but write there byte-by-byte | |
| 2. start update for N bytes, write N-x bytes, then write y bytes, y>x. | |
| You have to see that at the last write only x bytes were written, | |
| but y-x bytes were not. we may vary x a bit. good number would be | |
| 1, 128, 128Ki-128... | |
| 3. Try to start update for x bytes, write x bytes, then try to write more. | |
| Check that it is impossible to write more. |