poor performance when use qcow backend

[winglq]

Hi experts,
I have a nvme driver on hand and tested the performance of tcmu-runner.
The following is my test steps:

1. make filesystem on the nvme driver.

2. use qemu-img create a raw image on the new created filesystem.

3. use this image as lio backstore, and create the iscsi LUN.

4. login iscsi target

5. run fio on the new attached LUN.
   fio command:
   fio --name=nvme --file=/dev/sdn --rw=randwrite --bs=4k --iodepth=32 --size=10G --direct=1 --ioengine=libaio

6. the iops is 45k.

7. run fio on the mounted filesystem directly(nvme driver).

8. the iops is 220k.

Any idea why the iops down graded so many? How could I improve the performance?
Thanks.