Migration with ``drive-mirror`` + NBD will let quorum qcow2 image become larger

[Tao Xu <tao3.xu@intel.com>]

Hi,

I am using ``drive-mirror`` + NBD for live storage migration. But I find 
that if I use a qcow2 image(virtual size: 10 GiB, disk size: 1.8 GiB) as 
a child of quorum, then the destination image become larger(virtual 
size: 10 GiB, disk size: 10 GiB). However if I use a qcow2 image 
directly, then the destination image(virtual size: 10 GiB, disk size: 
1.8 GiB) will be equal to the source.

So I am wondering if my usage is wrong or it is expected with 
quorum+drive-mirror?

P.S. Detail:

1) [On *destination* Host]: qemu-img create -f qcow2 fedora32.qcow2 10G
Formatting 'fedora32.qcow2', fmt=qcow2 size=10737418240 
cluster_size=65536 lazy_refcounts=off refcount_bits=16

qemu-img info fedora32.qcow2
image: fedora32.qcow2
file format: qcow2
virtual size: 10 GiB (10737418240 bytes)
disk size: 196 KiB
cluster_size: 65536
Format specific information:
     compat: 1.1
     lazy refcounts: false
     refcount bits: 16
     corrupt: false

Boot the QEMU using:

disk_path=fedora32.qcow2
net_param="-netdev 
tap,id=hn0,vhost=off,br=br0,helper=/usr/local/libexec/qemu-bridge-helper 
-device rtl8139,id=e0,netdev=hn0"
cmdline="qemu-system-x86_64 \
-enable-kvm \
-m 2G -smp 4 -qmp stdio -bios OVMF.fd \
-monitor telnet:127.0.0.1:4444,nowait,server -vnc :7 -rtc base=utc \
-cpu host -device cirrus-vga,id=video0,bus=pci.0,addr=0x2 \
-device piix3-usb-uhci,id=usb,bus=pci.0,addr=0x1.0x2 \
-device usb-tablet,id=input0,bus=usb.0,port=1 $net_param \
-drive if=none,id=parent0,file.filename=$disk_path,driver=qcow2 \
-incoming tcp:0:8888"
exec $cmdline

[On *destination* QEMU]:
{'execute':'qmp_capabilities'}
{'execute': 'nbd-server-start', 'arguments': {'addr': {'type': 'inet', 
'data': {'host': '192.168.0.33', 'port': '8889'} } } }
{'execute': 'nbd-server-add', 'arguments': {'device': 'parent0', 
'writable': true } }

2) [On *source* Host]:

Boot the QEMU using:

disk_path=fedora32.qcow2
net_param="-netdev 
tap,id=hn0,vhost=off,br=br0,helper=/usr/local/libexec/qemu-bridge-helper 
-device rtl8139,id=e0,netdev=hn0"
cmdline="qemu-system-x86_64 \
-enable-kvm \
-m 2G -smp 4 -qmp stdio -bios OVMF.fd \
-monitor telnet:127.0.0.1:4444,nowait,server -vnc :7 -rtc base=utc \
-cpu host -device cirrus-vga,id=video0,bus=pci.0,addr=0x2 \
-device piix3-usb-uhci,id=usb,bus=pci.0,addr=0x1.0x2 \
-device usb-tablet,id=input0,bus=usb.0,port=1 $net_param \
-drive 
if=virtio,id=colo-disk0,driver=quorum,vote-threshold=1,children.0.file.filename=$disk_path,children.0.driver=qcow2"
exec $cmdline

[On *source* QEMU]:

{'execute':'qmp_capabilities'}
{'execute': 'drive-mirror', 'arguments':{ 'device': 'colo-disk0', 
'job-id': 'resync', 'target': 'nbd://192.168.0.33:8889/parent0', 'mode': 
'existing', 'format': 'nbd', 'sync': 'full'} }

{"timestamp": {"seconds": 1589902560, "microseconds": 107418}, "event": 
"JOB_STATUS_CHANGE", "data": {"status": "created", "id": "resync"}}
{"timestamp": {"seconds": 1589902560, "microseconds": 107487}, "event": 
"JOB_STATUS_CHANGE", "data": {"status": "running", "id": "resync"}}
{"return": {}}
{"timestamp": {"seconds": 1589902721, "microseconds": 439095}, "event": 
"JOB_STATUS_CHANGE", "data": {"status": "ready", "id": "resync"}}
{"timestamp": {"seconds": 1589902721, "microseconds": 439194}, "event": 
"BLOCK_JOB_READY", "data": {"device": "resync", "len": 10739253248, 
"offset": 10739253248, "speed": 0, "type": "mirror"}}

3)[On *destination* Host]:
qemu-img info fedora32.qcow2
image: fedora32.qcow2
file format: qcow2
virtual size: 10 GiB (10737418240 bytes)
disk size: 10 GiB
cluster_size: 65536
Format specific information:
     compat: 1.1
     lazy refcounts: false
     refcount bits: 16
     corrupt: false
4)But if [On *source* Host] boot qemu using:

disk_path=fedora32.qcow2
net_param="-netdev 
tap,id=hn0,vhost=off,br=br0,helper=/usr/local/libexec/qemu-bridge-helper 
-device rtl8139,id=e0,netdev=hn0"
cmdline="qemu-system-x86_64 \
-enable-kvm \
-m 2G -smp 4 -qmp stdio -bios OVMF.fd \
-monitor telnet:127.0.0.1:4444,nowait,server -vnc :7 -rtc base=utc \
-cpu host -device cirrus-vga,id=video0,bus=pci.0,addr=0x2 \
-device piix3-usb-uhci,id=usb,bus=pci.0,addr=0x1.0x2 \
-device usb-tablet,id=input0,bus=usb.0,port=1 $net_param \
-drive if=virtio,id=parent0,file.filename=$disk_path,driver=qcow2"
exec $cmdline

Then [On *destination* Host]:

qemu-img info fedora32.qcow2
image: fedora32.qcow2
file format: qcow2
virtual size: 10 GiB (10737418240 bytes)
disk size: 1.8 GiB
cluster_size: 65536
Format specific information:
     compat: 1.1
     lazy refcounts: false
     refcount bits: 16
     corrupt: false