[libvirt-users] Create qcow2 v3 volumes via libvirt

[Gionatan Danti]

Hi all,
on a fully patched CentOS 7.4 x86-64, I see the following behavior:

- when creating a new volumes using vol-create-as, the resulting file is 
a qcow2 version 2 (compat=0.10) file. Example:

[root at gdanti-lenovo vmimages]# virsh vol-create-as default zzz.qcow2 
8589934592 --format=qcow2 --backing-vol /mnt/vmimages/centos6.img
Vol zzz.qcow2 created
[root at gdanti-lenovo vmimages]# file zzz.qcow2
zzz.qcow2: QEMU QCOW Image (v2), has backing file (path 
/mnt/vmimages/centos6.img), 8589934592 bytes
[root at gdanti-lenovo vmimages]# qemu-img info zzz.qcow2
image: zzz.qcow2
file format: qcow2
virtual size: 8.0G (8589934592 bytes)
disk size: 196K
cluster_size: 65536
backing file: /mnt/vmimages/centos6.img
backing file format: raw
Format specific information:
     compat: 0.10
     refcount bits: 16

- when creating a snapshot, the resulting file is a qcow2 version 3 
(comapt=1.1) file. Example:

[root at gdanti-lenovo vmimages]# virsh snapshot-create-as centos6left 
--disk-only --no-metadata snap.qcow2
Domain snapshot snap.qcow2 created
[root at gdanti-lenovo vmimages]# file centos6left.snap.qcow2
centos6left.snap.qcow2: QEMU QCOW Image (v3), has backing file (path 
/mnt/vmimages/centos6left.qcow2), 8589934592 bytes
[root at gdanti-lenovo vmimages]# qemu-img info centos6left.snap.qcow2
image: centos6left.snap.qcow2
file format: qcow2
virtual size: 8.0G (8589934592 bytes)
disk size: 196K
cluster_size: 65536
backing file: /mnt/vmimages/centos6left.qcow2
backing file format: qcow2
Format specific information:
     compat: 1.1
     lazy refcounts: false
     refcount bits: 16
     corrupt: false

 From what I know, this is a deliberate decision: compat=1.1 requires 
relatively recent qemu version, and creating a new volume play on the 
"safe side" of compatibility.

It is possible to create a new volume using qcow2 version 3 (compat=1.1) 
format *using libvirt/virsh* (I know I can do that via qemu-img)? Any 
drawback on using version 3 format?

Thanks.

-- 
Danti Gionatan
Supporto Tecnico
Assyoma S.r.l. - www.assyoma.it
email: g.danti at assyoma.it - info at assyoma.it
GPG public key ID: FF5F32A8