[Qemu-devel] What does VM SIZE means when entering qemu-img info xx.qcow2

[lampahome]

I have an qcow2 image and I mount it to /dev/nbd then write somethin.

Then I take internal snapshot in the qcow2.

I enter the command qemu-img info qcow2
It shows:
image: qcow2
file format: qcow2
virtual size: 30G
disk size:4M
xxxxx
Snapshot list:
ID, TAG, VM SIZE, DATE, VM CLOCK
1, snap1, 0, xxxx-xx-xx, 00:00:00

The field of "VM SIZE" is zero in my host machine.
What does it mean and why is it an zero?