I have image A & B, and A is backing file of B. After I mount A to /dev/nbd0 and I write from position 0~999 in nbd0.
Then create B and set A as backing file of B. I mount B on /dev/nbd1 and I can saw the data from pos:0~999 because A is B's backing file. That's reasonable. But I want to discard range 0~500 in B. I expect there's no data in 0~500 after discard and re-mount B next time. But the data is still in A. How can I discard range 0~500?
