[Qemu-devel] qcow2 performance improve [discuss]

[yang.bin18]

When use cluster_size=1M qcow2 newly created to test "32k rand write" in the 
VM,We get low performance within VM。But there are huge “write” requests in HOST 
which falls into qemu   stack of"perform_cow".


         We found three scene to use cow.

                1.  write cluster have snapshot 

                2.  write img has backing file 

                3.  two concurrent writes happen to the same unallocated 
cluster.Each write allocates separate cluster and writes data concurrently 。

                For scene 2 and 3，we want to check the necessity before cow.

                For scene 2 ,to check bs->backing,if false ,not cow . 

                For scene 3,if not concurrent writes happen to the same 
unallocated cluster,not cow.