[Qemu-devel] Cluster doesn't truncate after commit to backing file

[lampahome]

I have two image A and B, and A is the backing file of B

I write data in address addr  in A.
Then create B and set A as backing file of B.
Then I discard the same addr in B.
So the entry of addr in l2 table should be 1( also is
QCOW2_CLUSTER_ZERO_PLAIN)
, right?

But the addr cluster in A will fill with zero after I execute `qemu-img
commit B`

I thought it will truncate or decrease the ref count of the cluster
corresponding the addr, but it looks like just fill zero.

Is that the default behavior?