[Qemu-devel] How to discard one range which overlap with backing file and its children img?

[lampahome]

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?