[Qemu-devel] reduce write bandwidth of qcow2 driver while allocating new cluster

[Liu Qing]

Dear list,
    Recently I used fio to test qcow2 driver in the guest os, and found out
that when a new cluster is allocated the 4K IO will occupy 64K(default cluster
size) bandwith.
    From the code qcow2 driver will fill the unused part of new allocated
cluster with 0 in perform_cow. These 0s are set in qcow2_co_readv when the read
destination is not allocated and it has no backing file. Could I forbidden any
further write in copy_sectors if the copy source is not allocated and it has
no backing file? So only the requested data is written to the cluster. Function
copy_sectors is only used by perform_cow in the master branch.
    Do you think this change is reasonable? Thanks.