Is it normal to get bigger qcow2 image after blockcopy?

[Fangge Jin]

Hi guys🔄  ❓

Recently, I found that the disk size of qcow2 image get bigger(from 6.16G
to 8G in my test) after blockcopy.🔄  ❓
I'm not sure whether this is normal or not. Please help to check. Thanks.🔄
  ❓

BR,🔄  ❓
Fangge Jin🔄  ❓

Before blockcopy, check source image:🔄  ❓
  # qemu-img info -U
/var/lib/avocado/data/avocado-vt/images/jeos-27-x86_64.qcow2🔄  ❓
  image: /var/lib/avocado/data/avocado-vt/images/jeos-27-x86_64.qcow2🔄  ❓
  file format: qcow2🔄  ❓
  virtual size: 10 GiB (10737418240 bytes)🔄  ❓
  disk size: 6.16 GiB🔄  ❓
  cluster_size: 65536🔄  ❓
  Format specific information:🔄  ❓
      compat: 1.1🔄  ❓
      compression type: zlib🔄  ❓
      lazy refcounts: false🔄  ❓
      refcount bits: 16🔄  ❓
      corrupt: false🔄  ❓
      extended l2: false🔄  ❓
  Child node '/file':🔄  ❓
      filename: /var/lib/avocado/data/avocado-vt/images/jeos-27-x86_64.qcow2
🔄  ❓
      protocol type: file🔄  ❓
      file length: 6.16 GiB (6614810624 bytes)🔄  ❓
      disk size: 6.16 GiB🔄  ❓
      Format specific information:🔄  ❓
          extent size hint: 1048576🔄  ❓

After blockcopy, check target image:🔄  ❓
  # qemu-img info -U /var/lib/avocado/data/avocado-vt/images/copy.qcow2 🔄
❓
  image: /var/lib/avocado/data/avocado-vt/images/copy.qcow2🔄  ❓
  file format: qcow2🔄  ❓
  virtual size: 10 GiB (10737418240 bytes)🔄  ❓
  disk size: 6.64 GiB🔄  ❓
  cluster_size: 65536🔄  ❓
  Format specific information:🔄  ❓
      compat: 1.1🔄  ❓
      compression type: zlib🔄  ❓
      lazy refcounts: false🔄  ❓
      refcount bits: 16🔄  ❓
      corrupt: false🔄  ❓
      extended l2: false🔄  ❓
  Child node '/file':🔄  ❓
      filename: /var/lib/avocado/data/avocado-vt/images/copy.qcow2🔄  ❓
      protocol type: file🔄  ❓
      file length: 6.64 GiB (7126974464 bytes)🔄  ❓
      disk size: 6.64 GiB🔄  ❓

Guest disk xml:🔄  ❓
    <disk type='file' device='disk'>🔄  ❓
      <driver name='qemu' type='qcow2'/>🔄  ❓
      <source
file='/var/lib/avocado/data/avocado-vt/images/jeos-27-x86_64.qcow2'
index='2'/>🔄  ❓
      <backingStore/>🔄  ❓
      <target dev='vda' bus='virtio'/>🔄  ❓
      <alias name='virtio-disk0'/>🔄  ❓
      <address type='pci' domain='0x0000' bus='0x04' slot='0x00'
function='0x0'/>🔄  ❓
    </disk>🔄  ❓

Qemu command line:🔄  ❓
  -blockdev
'{"driver":"file","filename":"/var/lib/avocado/data/avocado-vt/images/jeos-27-x86_64.qcow2","node-name":"libvirt-1-storage","auto-read-only":true,"discard":"unmap"}'
\🔄  ❓
  -blockdev
'{"node-name":"libvirt-1-format","read-only":false,"driver":"qcow2","file":"libvirt-1-storage","backing":null}'
\🔄  ❓
  -device
'{"driver":"virtio-blk-pci","bus":"pci.4","addr":"0x0","drive":"libvirt-1-format","id":"virtio-disk0","bootindex":1}'
\🔄  ❓

Qemu monitor command grepped from libvirt log:🔄  ❓

{"execute":"blockdev-add","arguments":{"driver":"file","filename":"/var/lib/avocado/data/avocado-vt/images/copy.qcow2","node-name":"libvirt-2-storage","auto-read-only":true,"discard":"unmap"},"id":"libvirt-429"}
🔄  ❓

{"execute":"blockdev-create","arguments":{"job-id":"create-libvirt-2-format","options":{"driver":"qcow2","file":"libvirt-2-storage","size":10737418240,"cluster-size":65536}},"id":"libvirt-430"}
🔄  ❓

{"execute":"job-dismiss","arguments":{"id":"create-libvirt-2-format"},"id":"libvirt-432"}
🔄  ❓

{"execute":"blockdev-add","arguments":{"node-name":"libvirt-2-format","read-only":false,"driver":"qcow2","file":"libvirt-2-storage","backing":null},"id":"libvirt-433"}
🔄  ❓

{"execute":"blockdev-mirror","arguments":{"job-id":"copy-vda-libvirt-1-format","device":"libvirt-1-format","target":"libvirt-2-format","sync":"full","auto-finalize":true,"auto-dismiss":false},"id":"libvirt-434"}
🔄  ❓

{"execute":"transaction","arguments":{"actions":[{"type":"block-dirty-bitmap-add","data":{"node":"libvirt-2-format","name":"libvirt-tmp-activewrite","persistent":false,"disabled":false}}]},"id":"libvirt-443"}
🔄  ❓

{"execute":"job-complete","arguments":{"id":"copy-vda-libvirt-1-format"},"id":"libvirt-444"}
🔄  ❓