QCOW2 virtual size lowering

I'm trying to get the virtual size of a qcow2 container down so that it can fit in a openstack flavor with 10GB disk.

root@node-10:~# qemu-img info zztop.qcow 
image: zztop.qcow
file format: qcow2
virtual size: 80G (85899345920 bytes)
disk size: 2.6G
cluster_size: 65536
Format specific information:
compat: 1.1
lazy refcounts: false

I follow the guide here and fill the end of the disk with zeros then run qemu-img convert -O qcow2. The resulting qcow2 however retains the same disk size and virtual size.

How do I lower the virtual size of a qcow2 so that it can fit in a openstack flavor smaller than 80GB?

Solution 1:

If you want to shrink the virtual size, you need to use virt-resize

Solution 2:

1. You need to convert the qcow2 image to raw
   qemu-img convert -O raw guest.img guest.raw
2. Then resize the raw file
   qemu-img resize guest.raw 3G
3. Then convert it back to qcow2
   qemu-img convert -O qcow2 -o compat=0.10 guest.raw guest.img
4. Then run the guest and resize your file system