If you print the offset after a write has safely completed, then it should still work. The internal copy is idempotent, up to the number of bytes in the first 3

If you print the offset after a write has safely completed, then it should still work. The internal copy is idempotent, up to the number of bytes in the first 300M lines. In other words, assuming reasonable line lengths, you can safely re-do a copy operation. So if you crash, you may not have printed out the last offset, but you have recorded a recent offset. You can then just restart from that recent offset.